Witnesses who saw the new moon — even on Shabbos
The 39 melachos — and the rabbinic fences
For two months they may break Shabbos
Permissivethey may desecrate Shabbos —
→ Nissan and Tishrei
Then the messengers go as far as Surya; by them the festivals are set
A reason — or just context?
Rashi, Meiri: context
While the Mikdash stood: every month — for the korban
Permissivethey would desecrate even for all the months,
→ for all of them
Reasonto ensure the Korban Mussaf is offered.
The Mishnah looks back from after the Churban
Rosh Hashana 1:4
Chillul Shabbos for the Rosh Chodesh Eidim
חִלּוּל שַׁבָּת לְעֵדֵי הַחֹדֶשׁ
עַל שְׁנֵי חֳדָשִׁים מְחַלְּלִין אֶת הַשַּׁבָּת, עַל נִיסָן וְעַל תִּשְׁרֵי,
שֶׁבָּהֶן הַשְּׁלוּחִין יוֹצְאִין לְסוּרְיָא, וּבָהֶן מְתַקְּנִין אֶת הַמּוֹעֲדוֹת.
וּכְשֶׁהָיָה בֵית הַמִּקְדָּשׁ קַיָּם,
מְחַלְּלִין אַף עַל כֻּלָּן מִפְּנֵי תַקָּנַת הַקָּרְבָּן.