Halacha

הלכה א
עֵדִים שֶׁרָאוּ אֶת הַחֹדֶשׁ אִם הָיָה בֵּינֵיהֶם וּבֵין מָקוֹם שֶׁיֵּשׁ בּוֹ בֵּית דִּין מַהֲלַךְ לַיְלָה וָיוֹם אוֹ פָּחוֹת הוֹלְכִין וּמְעִידִין. וְאִם הָיָה בֵּינֵיהֶן יֶתֶר עַל כֵּן לֹא יֵלְכוּ שֶׁאֵין עֵדוּתָן אַחַר יוֹם שְׁלֹשִׁים מוֹעֶלֶת שֶׁכְּבָר נִתְעַבֵּר הַחֹדֶשׁ:
כסף משנה
1.
When witnesses see the new [moon], and there is a journey of the night and a day or less between them and the place where the court holds sessions,1The Rambam does not say "a one-day distance from Jerusalem," because after the destruction of the Temple, the High Court would sanctify the new month in other places. (See Rosh HaShanah 22b.) they should undertake the journey and testify. If the distance between them is greater, they should not undertake the journey. For the testimony [that they will deliver] after the thirtieth day will be of no consequence,2Note the apparent contradiction in Halachah 15. In the light of that halachah, it would appear that the Rambam's intent is that since the court endeavors to nullify such testimony, it is undesirable for the witnesses to come and offer it (Lechem Mishneh). since the month will already have been made full.

הלכה ב
עֵדִים שֶׁרָאוּ אֶת הַחֹדֶשׁ הֲרֵי אֵלּוּ הוֹלְכִין לְבֵית דִּין לְהָעִיד וַאֲפִלּוּ הָיָה שַׁבָּת שֶׁנֶּאֱמַר (ויקרא כג ד) "אֲשֶׁר תִּקְרְאוּ אֹתָם בְּמוֹעֲדָם" וְכָל מָקוֹם שֶׁנֶּאֱמַר מוֹעֵד דּוֹחֶה אֶת הַשַּׁבָּת. לְפִיכָךְ אֵין מְחַלְּלִין אֶלָּא עַל רֹאשׁ חֹדֶשׁ נִיסָן וְעַל רֹאשׁ חֹדֶשׁ תִּשְׁרֵי בִּלְבַד מִפְּנֵי תַּקָּנַת הַמּוֹעֲדוֹת. וּבִזְמַן שֶׁבֵּית הַמִּקְדָּשׁ קַיָּם מְחַלְּלִין עַל כֻּלָּן מִפְּנֵי קָרְבַּן מוּסָף שֶׁבְּכָל רֹאשׁ חֹדֶשׁ וְחֹדֶשׁ שֶׁהוּא דּוֹחֶה אֶת הַשַּׁבָּת:
כסף משנה
2.
The witnesses who see the new [moon] should journey to the court to testify even on the Sabbath, as [implied by Leviticus 23:2]: "[These are the festivals] you should proclaim in their season." Whenever [the Torah] uses the word "season," the Sabbath [prohibitions]3The Rambam is referring to the prohibitions associated with going beyond the Sabbath limits and the performance of any forbidden labors necessary for their journey - e.g., carrying food or weapons. may be overridden.
Therefore, [the Sabbath prohibitions] may be violated only for the sake of Rosh Chodesh Nisan and Rosh Chodesh Tishrei,4Significantly, the month of Sivan is not mentioned, although the holiday of Shavuot is celebrated then. The celebration of Shavuot depends, not on a particular day of the month, but on the conclusion of the counting of the Omer. Thus when Rosh Chodesh was determined by the testimony of witnesses, Shavuot could be celebrated on the fifth, sixth, or seventh of Sivan (Rosh HaShanah 6b). to commemorate the festivals in their proper season. In the era of the Temple, [the Sabbath prohibitions] were violated for the sake of every Rosh Chodesh, because of the musaf offering sacrificed on Rosh Chodesh,5See Hilchot Temidim UMusafim 7:1,4, Hilchot Korban Pesach 1:18. since its [sacrifice] supersedes the Sabbath prohibitions.6The sacrifice of any offerings associated with a fixed time supersedes the Sabbath prohibitions.

הלכה ג
כְּשֵׁם שֶׁמְּחַלְּלִין הָעֵדִים שֶׁרָאוּ אֶת הַחֹדֶשׁ אֶת הַשַּׁבָּת כָּךְ מְחַלְּלִין עִמָּהֶן הָעֵדִים שֶׁמְּזַכִּין אוֹתָן בְּבֵית דִּין אִם לֹא הָיוּ בֵּית דִּין מַכִּירִין אֶת הָרוֹאִין. וַאֲפִלּוּ הָיָה זֶה שֶׁמּוֹדִיעַ אוֹתָן לְבֵית דִּין עֵד אֶחָד הֲרֵי זֶה הוֹלֵךְ עִמָּהֶן וּמְחַלֵּל מִסָּפֵק שֶׁמָּא יִמָּצֵא אַחֵר וְיִצְטָרֵף עִמּוֹ:
כסף משנה
3.
Just as the witnesses who see the new [moon] should violate the Sabbath [to testify], so too, the witnesses who substantiate their credibility7I.e., the character witnesses mentioned in Chapter 2, Halachah 3. should violate [the Sabbath to accompany] them, if the court is not familiar with the witnesses.
Even if only a single individual can testify regarding the witnesses, he should accompany them and violate the Sabbath,8This law is based on an actual historical account. Rabbi Nehorai traveled to Usha together with witnesses to substantiate their credibility, in the hope of finding another person there to testify together with him (Rosh HaShanah 22b). because of the possibility9The Perush questions why the Sabbath laws are broken when there is only a possibility of performing a mitzvah, citing a similar situation - a baby born after sunset on Friday. He should not be circumcised on the following Sabbath, because there is a doubt whether circumcision should be performed on Friday or on the Sabbath (Hilchot Milah 1:12).
The subsequent commentaries offer several resolutions to this difficulty. Among them:
a) Circumcision can be performed at a later date, while the sanctification of the new moon cannot be postponed (Lechem Mishneh);
b) Traveling to Jerusalem to testify always involves violating the Sabbath laws with only a possibility of performing the mitzvah, for it is possible that the court will reject the witnesses' testimony (Rav David Arameah).
that they might encounter another individual who can [testify] together with him.

הלכה ד
הָיָה הָעֵד שֶׁרָאָה אֶת הַחֹדֶשׁ בְּלֵיל הַשַּׁבָּת חוֹלֶה מַרְכִּיבִין אוֹתוֹ עַל הַחֲמוֹר וַאֲפִלּוּ בַּמִּטָּה. וְאִם יֵשׁ לָהֶן אוֹרֵב בַּדֶּרֶךְ לוֹקְחִין הָעֵדִים בְּיָדָן כְּלֵי זַיִן. וְאִם הָיָה דֶּרֶךְ רְחוֹקָה לוֹקְחִים בְּיָדָם מְזוֹנוֹת. וַאֲפִלּוּ רָאוּהוּ גָּדוֹל וְנִרְאֶה לַכּל לֹא יֹאמְרוּ כְּשֵׁם שֶׁרְאִינוּהוּ אֲנַחְנוּ רָאוּהוּ אֲחֵרִים וְאֵין אָנוּ צְרִיכִין לְחַלֵּל אֶת הַשַּׁבָּת אֶלָּא כָּל מִי שֶׁיִּרְאֶה הַחֹדֶשׁ וְיִהְיֶה רָאוּי לְהָעִיד וְיִהְיֶה בֵּינוֹ וּבֵין הַמָּקוֹם שֶׁקָּבוּעַ בּוֹ בֵּית דִּין לַיְלָה וָיוֹם אוֹ פָּחוֹת מִצְוָה עָלָיו לְחַלֵּל אֶת הַשַּׁבָּת וְלֵילֵךְ וּלְהָעִיד:
כסף משנה
4.
When a witness who sighted the moon on Friday night is sick, he may be mounted on a donkey [and transported to the High Court].10Leading a healthy person on a donkey is not considered to be transporting him in violation of the Sabbath labors, because "a living being carries himself." If, however, the person is sick, this principle does not apply, and he is considered a burden, so that transporting him on a donkey involves a prohibition (Hilchot Shabbat 18:16, 20:1-2). Nevertheless, this prohibition is waived in the endeavor to sanctify the new moon. [Indeed,] even [if he is bedridden,] his bed [may be transported].11Rav David Arameah interprets this to mean that men are allowed to carry him to the court on his bed.
If an ambush awaits them on the way,12Rashi, Rosh HaShanah 22a, relates that the Sadducees would lie in ambush for the witnesses, with the intent of delaying their appearance in court until after the thirtieth day had passed. the witnesses may carry weapons. If the distance [to the court] is long, they may carry food.
Even if the moon was sighted [with a] large [crescent], and [one is certain that] it was also sighted by many others as well, he should not say, "Just as we saw [the moon], so did others, and there is no necessity for us to violate the Sabbath laws."13In his Commentary on the Mishnah (Rosh HaShanah 1:5), the Rambam explains that although the moon might be seen very clearly in one place, in other locales it might have appeared low on the horizon, or covered by clouds. Therefore, it is desirable for all potential witnesses to journey to Jerusalem to make sure that the new moon is sanctified in its proper time. Instead, every person who saw the new moon, who is fit to serve as a witness, and who is within a distance of the night and a day14I.e., there is sufficient time for the witness(es) to arrive at the High Court on the thirtieth day. or less is commanded to violate the Sabbath laws and go and testify.15In this vein, it is worthy to note a story related in Rosh HaShanah 1:6. Forty pairs of witnesses passed through Lod on their way to Jerusalem to testify regarding the sighting of the moon. Rabbi Akiva held them back, because he saw that their testimony would not stand up under cross-examination by the court. Rabban Gamliel rebuked him, explaining that this would prevent them from journeying to Jerusalem at a future date, when their testimony might be of consequence. (See the Rambam's Commentary on the Mishnah.)

הלכה ה
בָּרִאשׁוֹנָה הָיוּ מְקַבְּלִין עֵדוּת הַחֹדֶשׁ בְּכָל יוֹם שְׁלֹשִׁים. פַּעַם אַחַת נִשְׁתַּהוּ הָעֵדִים מִלָּבוֹא עַד בֵּין הָעַרְבַּיִם וְנִתְקַלְקְלוּ בַּמִּקְדָּשׁ וְלֹא יָדְעוּ מַה יַּעֲשׂוּ אִם יַעֲשׂוּ עוֹלָה שֶׁל בֵּין הָעַרְבַּיִם שֶׁמָּא יָבוֹאוּ הָעֵדִים וְאִי אֶפְשָׁר שֶׁיַּקְרִיבוּ מוּסַף הַיּוֹם אַחַר תָּמִיד שֶׁל בֵּין הָעַרְבַּיִם. עָמְדוּ בֵּית דִּין וְהִתְקִינוּ שֶׁלֹּא יִהְיוּ מְקַבְּלִים עֵדוּת הַחֹדֶשׁ אֶלָּא עַד הַמִּנְחָה כְּדֵי שֶׁיְּהֵא שָׁהוּת בַּיּוֹם לְהַקְרִיב מוּסָפִין וְתָמִיד שֶׁל בֵּין הָעַרְבַּיִם וְנִסְכֵּיהֶם:
כסף משנה
5.
Originally, [the High Court] would accept testimony regarding the new moon throughout the entire thirtieth day. Once the witnesses were delayed and did not come until the evening. This created confusion in the Temple, and [the priests] did not know what to do:16More specifically, Rosh HaShanah 30b states that the problem concerns the song to be sung by the Levites in connection with the sacrifice. Had there not been a difficulty with the song, the afternoon sacrifice could have been offered conditionally: If no witnesses come, the lamb would be considered the afternoon sacrifice; and if the witnesses do come, it would be considered part of the musaf offering (Rishon LeTzion).
Significantly, as reflected in the discussion in Rosh HaShanah (ibid.), the instance in question concerned the celebration of Rosh HaShanah, and not an ordinary Rosh Chodesh.
If they would offer the afternoon sacrifice, [a difficulty would result] if witnesses [in fact] came, for it is impossible to sacrifice the musaf offering [of Rosh Chodesh] after the daily afternoon sacrifice.17For once the afternoon sacrifice is offered, no other offerings may be sacrificed (Pesachim 58b; Hilchot Temidim UMusafim 1:3).
At that time, [the High] Court instituted the [following] edict: Testimony regarding the new moon would be accepted only until the time of minchah,18This refers to the time of the afternoon sacrifice, nine and a half seasonal hours (שעות זמניות) after sunrise (The Rambam's Commentary on the Mishnah, Rosh HaShanah 4:4, Hilchot Temidim UMusafim 1:3). so that there would be enough time in the daylight hours to offer the musaf sacrifices, the daily afternoon offering, and the wine libations [that accompany these sacrifices].

הלכה ו
וְאִם הִגִּיעַ מִנְחָה וְלֹא בָּאוּ עֵדִים עוֹשִׂין תָּמִיד שֶׁל בֵּין הָעַרְבַּיִם. וְאִם בָּאוּ עֵדִים מִן הַמִּנְחָה וּלְמַעְלָה נוֹהֲגִין אוֹתוֹ הַיּוֹם קֹדֶשׁ וּלְמָחָר קֹדֶשׁ וּמַקְרִיבִין מוּסָף לְמָחָר לְפִי שֶׁלֹּא הָיוּ מְקַדְּשִׁין אוֹתוֹ אַחַר מִנְחָה. מִשֶּׁחָרַב בֵּית הַמִּקְדָּשׁ הִתְקִין רַבָּן יוֹחָנָן בֵּן זַכַּאי וּבֵית דִּינוֹ שֶׁיִּהְיוּ מְקַבְּלִין עֵדוּת הַחֹדֶשׁ כָּל הַיּוֹם כֻּלּוֹ וַאֲפִלּוּ בָּאוּ עֵדִים יוֹם שְׁלֹשִׁים בְּסוֹף הַיּוֹם סָמוּךְ לִשְׁקִיעַת הַחַמָּה מְקַבְּלִין עֵדוּתָן וּמְקַדְּשִׁין יוֹם שְׁלֹשִׁים בִּלְבַד:
כסף משנה
6.
If the time of minchah arrives without witnesses having come, the daily afternoon offering should be sacrificed. If witnesses come after the time of minchah,19The Rambam is referring to Rosh HaShanah. this day should be observed as a holiday,20Indeed, the people would observe the day as a holiday from nightfall on the previous day, lest witnesses come and the new moon be sanctified (Rashi, Rosh HaShanah 30b). See Halachah 9, and Chapter 5, Halachah 8. and the following day should be observed as a holiday. The musaf offering, however, should be offered on the following day, because the new moon should not be sanctified after the time of minchah.
After the Temple was destroyed,21And there were no longer any difficulties concerning the sacrifices. Rabbi Yochanan ben Zakkai and his court22Rabban Yochanan ben Zakkai lived at the time of the Temple's destruction, and this decree was instituted shortly after that event. instituted a decree [requiring the High Court] to accept testimony regarding the new moon throughout the entire day. Even if witnesses come at the end of the thirtieth day before sunset,23The Lechem Mishneh and others have raised questions about the Rambam's choice of wording, based on his previous statements in Chapter 2, Halachah 9. Merkevet HaMishneh offers a simple resolution of this difficulty. Here we are speaking of the acceptance of the witnesses' testimony, and it is necessary to take time to cross-examine them. This process cannot be completed in the meager amount of time between sunset and the appearance of the stars. their testimony should be accepted and the thirtieth day alone should be considered a holiday.

הלכה ז
כְּשֶׁמְּעַבְּרִין בֵּית דִּין אֶת הַחֹדֶשׁ מִפְּנֵי שֶׁלֹּא בָּאוּ עֵדִים כָּל יוֹם שְׁלֹשִׁים הָיוּ עוֹלִין לְמָקוֹם מוּכָן וְעוֹשִׂין בּוֹ סְעֻדָּה בְּיוֹם אֶחָד וּשְׁלֹשִׁים שֶׁהוּא רֹאשׁ חֹדֶשׁ. וְאֵין עוֹלִין לְשָׁם בַּלַּיְלָה אֶלָּא בַּנֶּשֶׁף קֹדֶם עֲלוֹת הַשֶּׁמֶשׁ. וְאֵין עוֹלִין לִסְעֻדָּה זוֹ פָּחוֹת מֵעֲשָׂרָה. וְאֵין עוֹלִין לָהּ אֶלָּא בְּפַת דָּגָן וְקִטְנִית. וְאוֹכְלִין בְּעֵת הַסְּעֻדָּה. וְזוֹ הִיא סְעֻדַּת מִצְוָה שֶׁל עִבּוּר הַחֹדֶשׁ הָאֲמוּרָה בְּכָל מָקוֹם:
כסף משנה
7.
When the court would make a month full because the witnesses did not appear on the thirtieth day at all, they would ascend to a previously designated place and hold a feast on the thirty-first day, which would be Rosh Chodesh.24The purpose of holding this feast would be to publicize that the month had become full.
They would not ascend there during the [thirty-first] night, but rather at daybreak, before the sun rose.25Sanhedrin 70a relates that the participants in this meal would ascend in a time that is neither day nor night. Thus, the Rambam interprets this as referring to the time between the appearance of the first rays of the sun and sunrise itself. Rashi and others maintain that the participants would ascend in the evening, between sunset and the appearance of the stars. No fewer than ten men would ascend [to participate] in this meal. They would not hold this meal without bread made from grain and legumes, of which they would partake during the meal. This is the intent of all the sources that refer to the feast associated with the mitzvah of the full month.

הלכה ח
בָּרִאשׁוֹנָה כְּשֶׁהָיוּ בֵּית דִּין מְקַדְּשִׁין אֶת הַחֹדֶשׁ הָיוּ מַשִּׂיאִין מַשּׂוּאוֹת בְּרָאשֵׁי הֶהָרִים כְּדֵי שֶׁיֵּדְעוּ הָרְחוֹקִים. מִשֶּׁקִּלְקְלוּ הַכּוּתִים שֶׁהָיוּ מַשִּׂיאִין מַשּׂוּאוֹת כְּדֵי לְהַטְעוֹת אֶת הָעָם הִתְקִינוּ שֶׁיְּהוּ שְׁלוּחִים יוֹצְאִין וּמוֹדִיעִין לָרַבִּים. וּשְׁלוּחִים אֵלּוּ אֵינָן מְחַלְּלִין אֶת יוֹם טוֹב וְלֹא אֶת יוֹם הַכִּפּוּרִים וְאֵין צָרִיךְ לוֹמַר שַׁבָּת שֶׁאֵין מְחַלְּלִין אֶת הַשַּׁבָּת לְקַיְּמוֹ אֶלָּא לְקַדְּשׁוֹ בִּלְבַד:
כסף משנה
8.
Originally, when the court would sanctify the new moon, they would light bonfires26The fires would be lit in the evening of the thirty-first night. on the mountaintops to notify the people in distant places.27There are mountains surrounding Jerusalem on all sides. The bonfires on these mountains could be seen by people on further mountains, until in a short amount of time people throughout Eretz Yisrael could be notified that the new moon had been sanctified. When the Samaritans began conducting themselves in a debased manner and would light bonfires [at the wrong times] to confuse the people,28The Samaritans lived between Jerusalem and the Galilee. At one point, they desired to disrupt the calculation of the calendar and would light bonfires on the evening before the thirty-first day, regardless of whether or not the moon was sighted. The people in the Galilee would think that the chain of bonfires had begun in Jerusalem (Rosh HaShanah 22b). [the Sages] instituted the practice of having messengers journey to notify the people.29The Jerusalem Talmud (Rosh HaShanah 2:1) states that Rabbi Yehudah HaNasi was the one who nullified the bonfires, and substituted messengers instead. This, however, is somewhat difficult to accept because, as the Rambam mentions in Halachah 9, there were times when the messengers were sent out while the Temple was standing. Perhaps the intent is that until the time of Rabbi Yehudah HaNasi there were times when both messengers were sent out and bonfires were lit. From his time onward, the bonfires were no longer used.
These messengers may not violate [the sanctity of] the holidays, nor of Yom Kippur [by traveling on these days]. Needless to say, they may not violate [the sanctity of] the Sabbath. For one may not violate the Sabbath to uphold [the sanctification of] the new month, only to [actually] sanctify it.

הלכה ט
עַל שִׁשָּׁה חֳדָשִׁים הָיוּ שְׁלוּחִים יוֹצְאִין. עַל נִיסָן מִפְּנֵי הַפֶּסַח. וְעַל אָב מִפְּנֵי הַתַּעֲנִית. וְעַל אֱלוּל מִפְּנֵי רֹאשׁ הַשָּׁנָה. כְּדֵי שֶׁיֵּשְׁבוּ מְצַפִּין בְּיוֹם שְׁלֹשִׁים לֶאֱלוּל אִם נוֹדַע לָהֶם שֶׁקִּדְּשׁוּ בֵּית דִּין יוֹם שְׁלֹשִׁים נוֹהֲגִים אוֹתוֹ הַיּוֹם קֹדֶשׁ בִּלְבַד. וְאִם לֹא נוֹדַע לָהֶם נוֹהֲגִים יוֹם שְׁלֹשִׁים קֹדֶשׁ וְיוֹם אֶחָד וּשְׁלֹשִׁים קֹדֶשׁ עַד שֶׁיָּבוֹאוּ לָהֶם שְׁלוּחֵי תִּשְׁרֵי. וְעַל תִּשְׁרֵי מִפְּנֵי תַּקָּנַת הַמּוֹעֲדוֹת. וְעַל כִּסְלֵו מִפְּנֵי חֲנֻכָּה. וְעַל אֲדָר מִפְּנֵי הַפּוּרִים. וּבִזְמַן שֶׁבֵּית הַמִּקְדָּשׁ קַיָּם יוֹצְאִין אַף עַל אִיָּר מִפְּנֵי פֶּסַח קָטָן:
כסף משנה
9.
Messengers are sent out [to inform the people] for six months: For Nisan, because of Pesach. For Av, because of the fast [of Tish'ah B'Av].30Rosh HaShanah 18b relates that in that era, fasting on the other commemorative fasts - the tenth of Tevet and the seventeenth of Tammuz - was not mandatory. Therefore, it was not necessary to send out messengers for these months. For Elul, because of Rosh HaShanah - i.e., so they could wait in readiness on the thirtieth day of Elul.31The Rambam maintains that the people would observe Rosh HaShanah for two days, because of the doubt. Thus, if the people did not know when the month of Elul began, they would have to observe three days (Lechem Mishneh). If it became known to them that the High Court had sanctified the thirtieth day, they would observe only that day as a holiday. If it did not become known to them,32This is speaking about the villages in the immediate vicinity of Jerusalem. they would observe both the thirtieth and the thirty-first days [as Rosh HaShanah] until the messengers of Tishrei arrived.
For Tishrei, because of the festivals. For Kislev, because of Chanukah. For Adar, because of Purim.33The commentaries note that Rosh HaShanah 19b also mentions the possibility of the messengers' being sent out in the second month of Adar if a leap year is declared. This, however, is an infrequent event, because generally, the decision to make the year a leap year was not made at the last moment (Aruch HaShulchan). While the Beit HaMikdash was standing, [messengers] also would be sent out for the month of Iyar, because of the small Pesach.34Pesach Sheni, when all those who had not offered a sacrifice on the first Pesach were given a further opportunity to bring this offering.
Messengers were not sent out for the month of Sivan, because the celebration of Shavuot is not dependent on the date of the month, but on the Counting of the Omer (Perush).

הלכה י
שְׁלוּחֵי נִיסָן וּשְׁלוּחֵי תִּשְׁרֵי אֵין יוֹצְאִין אֶלָּא בְּיוֹם רֹאשׁ חֹדֶשׁ אַחַר שֶׁתַּעֲלֶה הַשֶּׁמֶשׁ עַד שֶׁיִּשְׁמְעוּ מִפִּי בֵּית דִּין מְקֻדָּשׁ. וְאִם קִדְּשׁוּ בֵּית דִּין בְּסוֹף יוֹם תִּשְׁעָה וְעֶשְׂרִים כְּמוֹ שֶׁאָמַרְנוּ וְשָׁמְעוּ מִפִּי בֵּית דִּין מְקֻדָּשׁ יוֹצְאִין מִבָּעֶרֶב. וּשְׁלוּחֵי שְׁאָר הַשִּׁשָּׁה חֳדָשִׁים יֵשׁ לָהֶם לָצֵאת מִבָּעֶרֶב אַחַר שֶׁנִּרְאָה הַיָּרֵחַ אַף עַל פִּי שֶׁעֲדַיִן לֹא קִדְּשׁוּ בֵּית דִּין אֶת הַחֹדֶשׁ הוֹאִיל וְנִרְאֶה הַחֹדֶשׁ יָצְאוּ שֶׁהֲרֵי לְמָחָר בְּוַדַּאי מְקַדְּשִׁין אוֹתוֹ בֵּית דִּין:
כסף משנה
10.
[Even when the moon was clearly sighted on the previous night,] the messengers for the months of Nisan and Tishrei would not depart35The messengers for the month of Tishrei could travel only 2000 cubits beyond Jerusalem (or 4000 cubits, if they made an eruv t'chumim) on Rosh HaShanah, because of the sanctity of that day (Perush). until the sun rose and they heard the court pronounce, "It has been sanctified."36Since the holidays observed in these months were prescribed by the Torah itself, this further stringency was required (ibid.). If, however, the court sanctified the moon at the conclusion of the twenty-ninth day, [which is possible,] as we have explained,37See Chapter 2, Halachah 9. and the messengers heard the court pronounce it sanctified, they might depart that evening.
The messengers for the other months, by contrast, may depart in the evening after the moon has been sighted. Although the court has not sanctified the new month, since the new moon has been sighted, they may depart, for the court will surely sanctify the new month on the following day.

הלכה יא
כָּל מָקוֹם שֶׁהָיוּ הַשְּׁלוּחִין מַגִּיעִין הָיוּ עוֹשִׂין אֶת הַמּוֹעֲדוֹת יוֹם טוֹב אֶחָד כַּכָּתוּב בַּתּוֹרָה. וּמְקוֹמוֹת הָרְחוֹקִים שֶׁאֵין הַשְּׁלוּחִים מַגִּיעִין אֲלֵיהֶם הָיוּ עוֹשִׂין שְׁנֵי יָמִים מִפְּנֵי הַסָּפֵק לְפִי שֶׁאֵינָם יוֹדְעִים יוֹם שֶׁקָּבְעוּ בּוֹ בֵּית דִּין אֶת הַחֹדֶשׁ אֵי זֶה יוֹם הוּא:
כסף משנה
11.
Wherever these messengers would arrive [before the celebration of the festivals], the holidays would be observed for [only] one day, as prescribed by the Torah. In the distant places, which the messengers would not reach [before the celebration of the festivals],38The observance of an additional day of the holidays is described in greater detail in Chapter 5. the holidays would be observed for two days because of the doubt [involved]. For they would not know the day on which the High Court established the new month.39Thus, if the thirtieth day following Rosh Chodesh Adar fell on the Sabbath, they would not know whether Pesach (the fifteenth day of the following month) would be celebrated on the Sabbath or on Sunday.

הלכה יב
יֵשׁ מְקוֹמוֹת שֶׁהָיוּ מַגִּיעִין אֲלֵיהֶם שְׁלוּחֵי נִיסָן וְלֹא הָיוּ מַגִּיעִין לָהֶן שְׁלוּחֵי תִּשְׁרֵי. וּמִן הַדִּין הָיָה שֶׁיַּעֲשׂוּ פֶּסַח יוֹם אֶחָד שֶׁהֲרֵי הִגִּיעוּ לָהֶן שְׁלוּחִין וְיָדְעוּ בְּאֵי זֶה יוֹם נִקְבַּע רֹאשׁ חֹדֶשׁ. וְיַעֲשׂוּ יוֹם טוֹב שֶׁל חַג הַסֻּכּוֹת שְׁנֵי יָמִים שֶׁהֲרֵי לֹא הִגִּיעוּ אֲלֵיהֶן הַשְּׁלוּחִין. וּכְדֵי שֶׁלֹּא לַחֲלֹק בַּמּוֹעֲדוֹת הִתְקִינוּ חֲכָמִים שֶׁכָּל מָקוֹם שֶׁאֵין שְׁלוּחֵי תִּשְׁרֵי מַגִּיעִין שָׁם עוֹשִׂין שְׁנֵי יָמִים אֲפִלּוּ יוֹם טוֹב שֶׁל עֲצֶרֶת:
כסף משנה
12.
There are places where the messengers [sent out for] Nisan would arrive [in time for the celebration of Pesach], but the messengers [sent out for] Tishrei would not arrive [in time for the celebration of Sukkot].40For, as explained in the following halachah, the messengers sent out for Tishrei would not travel on Rosh HaShanah or on Yom Kippur.
In the brackets, we mention the holiday of Sukkot, but not Yom Kippur. For Yom Kippur was never observed by a community at large for two days. Rosh HaShanah 21a relates that Rabbah would fast for two days, but this was a stringency that he accepted on himself as an individual. The people in the diaspora would fast ten days after the thirtieth day following Rosh Chodesh Elul, for the court always endeavored to make Elul a month of 29 days. (See Or Sameach.)
According to [the letter of] the law, it would have been appropriate for them to observe Pesach for one day, since the messengers reached them and informed them when Rosh Chodesh had been established, and for them to observe Sukkot for two days, since the messengers had not reached them. Nevertheless, so that there would be no difference between the festivals, the Sages instituted the ruling that two days were celebrated [for all holidays] in all places that were not reached by the messengers for Tishrei. [This includes] even the festival of Shavuot.41As mentioned several times, Shavuot is unique, because its observance is dependent, not on a date in the month of Sivan, but on the Counting of the Omer, which begins after Pesach. Thus, even Jews living in the outer reaches of the diaspora were able to know when Shavuot was to be observed.

הלכה יג
וְכַמָּה בֵּין שְׁלוּחֵי נִיסָן לִשְׁלוּחֵי תִּשְׁרֵי. שְׁנֵי יָמִים. שֶׁשְּׁלוּחֵי תִּשְׁרֵי אֵינָן מְהַלְּכִין בְּאֶחָד בְּתִשְׁרֵי מִפְּנֵי שֶׁהוּא יוֹם טוֹב וְלֹא בַּעֲשִׂירִי בּוֹ מִפְּנֵי שֶׁהוּא יוֹם כִּפּוּר:
כסף משנה
13.
[How many more] days do the messengers for Nisan journey than the messengers for Tishrei? Two. For the messengers for Tishrei do not travel on the first of Tishrei, because it is a holiday, nor on the tenth, because it is Yom Kippur.42There is also the possibility of additional days, if there are more Sabbaths in Tishrei than in Nisan.

הלכה יד
אֵין הַשְּׁלוּחִין צְרִיכִין לִהְיוֹתָן שְׁנַיִם אֶלָּא אֲפִלּוּ אֶחָד נֶאֱמָן. וְלֹא שָׁלִיחַ בִּלְבַד אֶלָּא אֲפִלּוּ תַּגָּר מִשְּׁאָר הָעָם שֶׁבָּא כְּדַרְכּוֹ וְאָמַר אֲנִי שָׁמַעְתִּי מִפִּי בֵּית דִּין שֶׁקִּדְּשׁוּ אֶת הַחֹדֶשׁ בְּיוֹם פְּלוֹנִי נֶאֱמָן וּמְתַקְּנִין אֶת הַמּוֹעֲדוֹת עַל פִּיו. שֶׁדָּבָר זֶה דָּבָר הֶעָשׂוּי לְהִגָּלוֹת הוּא וְעֵד אֶחָד כָּשֵׁר נֶאֱמָן עָלָיו:
כסף משנה
14.
There is no need for there to be two messengers. Even a single individual's [statements] are believed. Moreover, this does not apply to messengers alone. Even when a [traveling] merchant of no particular distinction passes through on his journey, and says: "I heard from the court43From the Rambam's wording, it would appear that it is necessary that the merchant receive his information from the High Court directly; hearing from others is not sufficient. that it sanctified the new month on such and such a date," his statements are believed, and the [celebration of] the festivals is arranged accordingly.
[The rationale is that] this is a matter that will [eventually] be revealed.44Rosh HaShanah 22b states that people will not lie about a matter when it is certain that eventually the truth will be revealed. Therefore, the testimony of a single acceptable witness is sufficient.45The commentaries compare the Rambam's statements here with those at the conclusion of Hilchot Gerushin. In the latter source, he employs the same rationale - that the truth of the matter will eventually be revealed - to explain why leniency is shown concerning testimony given to determine that a woman's husband has died, which enables her to remarry. In that instance, however, even greater leniency is shown, and the court accepts the testimony of witnesses who are otherwise unacceptable. The commentaries explain that the greater leniency is granted in consideration of the agonizing situation of the widow.

הלכה טו
בֵּית דִּין שֶׁיָּשְׁבוּ כָּל יוֹם שְׁלֹשִׁים וְלֹא בָּאוּ עֵדִים וְהִשְׁכִּימוּ בַּנֶּשֶׁף וְעִבְּרוּ אֶת הַחֹדֶשׁ כְּמוֹ שֶׁבֵּאַרְנוּ בְּפֶרֶק זֶה. וְאַחַר אַרְבָּעָה אוֹ חֲמִשָּׁה יָמִים בָּאוּ עֵדִים רְחוֹקִים וְהֵעִידוּ שֶׁרָאוּ אֶת הַחֹדֶשׁ בִּזְמַנּוֹ שֶׁהוּא לֵיל שְׁלֹשִׁים. וַאֲפִלּוּ בָּאוּ בְּסוֹף הַחֹדֶשׁ. מְאַיְּמִין עֲלֵיהֶן אִיּוּם גָּדוֹל וּמַטְרִיפִים אוֹתָם בִּשְׁאֵלוֹת וּמַטְרִיחִין עֲלֵיהֶן בִּבְדִיקוֹת וּמְדַקְדְּקִין בְּעֵדוּת וּמִשְׁתַּדְּלִין בֵּית דִּין שֶׁלֹּא יְקַדְּשׁוּ חֹדֶשׁ זֶה הוֹאִיל וְיָצָא שְׁמוֹ מְעֻבָּר:
כסף משנה
15.
[The following rules apply when] the court held session throughout the entire thirtieth day, but witnesses did not arrive, [the judges] arose early in the morning and made the month full, as we stated previously in this chapter,46See Halachah 7. and after four or five days witnesses came from distant places and testified that they had sighted the moon at the appropriate time, the thirtieth night. [Indeed, the same principles apply if the witnesses] come at the end of the month.
We unnerve them in a very intimidating matter, and we seek to disorient them with queries. We cross-examine them very thoroughly and are extremely precise regarding their testimony. [For] the court endeavors not to sanctify this month, since it has already been declared full.47The Rambam's statements in this and the following four halachot are based on his interpretation of Rosh HaShanah 20a. Significantly, his understanding of that passage differs from that of Rashi.

הלכה טז
וְאִם עָמְדוּ הָעֵדִים בְּעֵדוּתָן וְנִמְצֵאת מְכֻוֶּנֶת וַהֲרֵי הָעֵדִים אֲנָשִׁים יְדוּעִים וּנְבוֹנִים וְנֶחְקְרָה הָעֵדוּת כָּרָאוּי. מְקַדְּשִׁין אוֹתוֹ וְחוֹזְרִין וּמוֹנִין לְאוֹתוֹ הַחֹדֶשׁ מִיּוֹם שְׁלֹשִׁים הוֹאִיל וְנִרְאֶה הַיָּרֵחַ בְּלֵילוֹ:
כסף משנה
16.
If the witnesses remain steadfast in their testimony, if it is compatible [according to the calculations of the court], if the witnesses are men whose character is well known, they are men of understanding, and if their testimony was scrutinized in a proper way - the moon is sanctified [retroactively]. We recalculate the dates of the month beginning from the thirtieth day [after the previous Rosh Chodesh], since the moon was sighted [on the appropriate] night.

הלכה יז
וְאִם הֻצְרְכוּ בֵּית דִּין לְהַנִּיחַ חֹדֶשׁ זֶה מְעֻבָּר כְּשֶׁהָיָה קֹדֶם שֶׁיָּבוֹאוּ הָעֵדִים אֵלּוּ מַנִּיחִין. וְזֶה הוּא שֶׁאָמְרוּ מְעַבְּרִין אֶת הַחֹדֶשׁ לְצֹרֶךְ. וְיֵשׁ מִן הַחֲכָמִים הַגְּדוֹלִים מִי שֶׁחוֹלֵק בְּדָבָר זֶה וְאוֹמֵר לְעוֹלָם אֵין מְעַבְּרִין אֶת הַחֹדֶשׁ לְצֹרֶךְ. הוֹאִיל וּבָאוּ עֵדִים מְקַדְּשִׁין וְאֵין מְאַיְּמִין עֲלֵיהֶן:
כסף משנה
17.
If it was necessary for the court to leave the month full, as it was before the witnesses came, it is left as it is. This is what is meant by the statement,48Rosh HaShanah, op. cit. "The month is made full out of necessity49Note another interpretation of the expression, "The month is made full out of necessity," in Chapter 18, Halachah 11.." Some great Sages differ concerning this matter and maintain that the month is never made full out of necessity. Rather, if the witnesses come, the month is sanctified and they are not intimidated at all50This opinion is also mentioned in the above passage..

הלכה יח
יֵרָאֶה לִי שֶׁאֵין מַחֲלֹקֶת הַחֲכָמִים בְּדָבָר זֶה אֶלָּא בִּשְׁאָר הֶחֳדָשִׁים חוּץ מִן נִיסָן וְתִשְׁרֵי. אוֹ בְּעֵדֵי נִיסָן וְתִשְׁרֵי שֶׁבָּאוּ אַחַר שֶׁעָבְרוּ הָרְגָלִים. שֶׁכְּבָר נַעֲשָׂה מַה שֶּׁנַּעֲשָׂה וְעָבַר זְמַן הַקָּרְבָּנוֹת וּזְמַן הַמּוֹעֲדוֹת. אֲבָל אִם בָּאוּ הָעֵדִים בְּנִיסָן וְתִשְׁרֵי קֹדֶם חֲצִי הַחֹדֶשׁ מְקַבְּלִין עֵדוּתָן וְאֵין מְאַיְּמִין עֲלֵיהֶן כְּלָל. שֶׁאֵין מְאַיְּמִין עַל עֵדִים שֶׁהֵעִידוּ עַל הַחֹדֶשׁ שֶׁרָאוּהוּ בִּזְמַנּוֹ כְּדֵי לְעַבְּרוֹ:
כסף משנה
18.
It appears to me51Generally, when the Rambam uses the expression "It appears to me" in the Mishneh Torah, he is introducing an original interpretation. In this instance, the resolution offered by the Rambam is also suggested by Abbaye in the above passage. Nevertheless, there is a certain justification for the use of the phrase, because there are other interpretations offered by the Talmud, and no conclusion is reached. Therefore, the Rambam's choice of the interpretation to follow was motivated by his own comprehension of the passage. that there is a difference of opinion among the Sages regarding this matter only in [the following circumstances]:
a) The other months besides Nisan and Tishrei, or
b) When witnesses come in Nisan and Tishrei after the holidays have passed, [all the festive practices] have already been observed, and the time to offer the sacrifices and observe the festivals has passed.
When, however, the witnesses come in Tishrei and in Nisan before the middle of the month,52When Pesach and Sukkot are celebrated. One might ask why consideration was not given to the observance of Yom Kippur on the tenth of Tishrei.
The Or Sameach offers the following resolution: Yom Kippur was observed for only one day in the outlying diaspora. The communities would assume that the moon had been sighted at its appropriate time, and they would observe Rosh HaShanah on the thirtieth day after Rosh Chodesh Elul, and Yom Kippur ten days afterwards. Thus, by accepting the witnesses' testimony, the court will have assured that a large portion of the Jewish people observed Yom Kippur on the proper date.
Although this explanation has its advantages, it does not address itself to the observance of Yom Kippur in Eretz Yisrael, or to the sacrifices that had to be offered on that sacred day in the Temple.
their testimony is accepted, and no attempt is made to intimidate them at all. For we do not attempt to intimidate witnesses who testify that they sighted at the appropriate time so that the month will be full.

הלכה יט
אֲבָל מְאַיְּמִין עַל עֵדִים שֶׁנִּתְקַלְקְלָה עֵדוּתָן וַהֲרֵי הַדָּבָר נוֹטֶה וּגְנַאי שֶׁלֹּא תִּתְקַיֵּם הָעֵדוּת וְיִתְעַבֵּר הַחֹדֶשׁ. מְאַיְּמִין עֲלֵיהֶן כְּדֵי שֶׁתִּתְקַיֵּם הָעֵדוּת וְיִתְקַיֵּם הַחֹדֶשׁ בִּזְמַנּוֹ. וְכֵן אִם בָּאוּ עֵדִים לְהָזִים אֶת הָעֵדִים שֶׁרָאוּהוּ בִּזְמַנּוֹ קֹדֶם שֶׁקִּדְּשׁוּהוּ בֵּית דִּין הֲרֵי אֵלּוּ מְאַיְּמִין עַל הַמְּזִימִין עַד שֶׁלֹּא תִּתְקַיֵּם הַהֲזָמָה וְיִתְקַדֵּשׁ הַחֹדֶשׁ בִּזְמַנּוֹ:
כסף משנה
19.
We do, however, intimidate witnesses whose testimony has been disputed, and it appears that their testimony will not be upheld, and the month will be made full. We pressure them so that their testimony will be upheld and the month will be sanctified in its proper time. Similarly, when, before the new month has been sanctified, witnesses come to nullify53The Hebrew term used, lehazim, means to nullify their testimony by stating that, at the time they claimed to have seen the moon, they were in a different place from the place where they claimed to have sighted it. the testimony of the witnesses who saw the moon at its appropriate time, [the court] intimidates the witnesses who want to nullify the original testimony, so that the challenge will not be accepted and the new month will be sanctified at its appropriate time.54It would appear that, in both of the instances mentioned in this halachah, this law applies only when, according to the court's calculations, the moon should have been sighted on the thirtieth night.

זמנים הלכות קידוש החודש פרק ג
Zemanim Kiddush HaChodesh Chapter 3