The Rambam · Sefer Zemanim
הלכות קידוש החודש ג׳Sanctification of the New Month 3
Chapter 3 of 19 in Sanctification of the New Month — 19 halachot
Halacha 1א׳
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:
Witnesses that see the month, if there is between them and the location of the court a distance of a night and day's journey or less, they go and testify. And if there is between them more than this, they don't go for their testimony after the 30th day doesn't help since the month has already been made pregnant (into a 30 day month).
Halacha 2ב׳
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:
Witnesses that saw the month go to the court to testify, even on Shabbat, as it is said (Leviticus 23:4) "that you call them at their appointed time" and anywhere it is said "appointed time" the Shabbat is pushed off. Thus, we only violate the Shabbat for the first of the month of Nissan which contains the festival of Passover and for the first of the month of Tishrei which contains the festivals of the Day of Atonement and Tabernacles in order to fix the festivals which are described with the term "appointed time". And in a time when the Holy Temple exists, we violate the Shabbat for all the months because of the additional sacrifice of every first of the month which pushes off the Shabbat.
Halacha 3ג׳
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:
Just as the witnesses that saw the month desecrate the Shabbat, so too the witnesses who justify them to the court desecrate with them if the court doesn't recognise the ones who saw. And even if the one who informs the court of them is a single witness, he goes with them and desecrates in case he will find another and join up with him.
Mishneh Torah, Torat Emet 363 edition, via Sefaria — public domain
English: Sefaria Community Translation — CC0