Kiddush HaChodesh
witnesses see the new moon · Beis Din declares it · messengers carry the news
A month is 29 days or 30
Messengers sent out to tell everyone
Six months the messengers go out
Permissivethe messengers go out — for Nisan,
Nisan — Pesach
Not Sivan: Shavuos is counted from Pesach
Av — the fast
Permissivefor Av,
Elul — Rosh Hashanah · Tishrei — the festivals
Permissivefor Elul,
Elul was taken as 29 days — so two days of Rosh Hashanah
Permissivefor Tishrei,
Kislev — Chanukah · Adar — Purim
Permissivefor Kislev,
Permissiveand for Adar,
Three of the six are days set by Chazal
While the Beis HaMikdash stood: Iyar too — for Pesach Sheni
Permissivethey would go out for Iyar as well,
Pesach Sheni: 14 Iyar
Was Tisha B'Av kept in the second Beis HaMikdash?
Rosh Hashana 1:3
Monthly Messengers
עַל אֵילוּ חֳדָשִׁים יוֹצְאִים הַשְּׁלוּחִין
עַל שִׁשָּׁה חֳדָשִׁים הַשְּׁלוּחִין יוֹצְאִין:
עַל נִיסָן מִפְּנֵי הַפֶּסַח,
עַל אָב מִפְּנֵי הַתַּעֲנִית,
עַל אֱלוּל מִפְּנֵי רֹאשׁ הַשָּׁנָה,
עַל תִּשְׁרֵי מִפְּנֵי תַקָּנַת הַמּוֹעֲדוֹת,
עַל כִּסְלֵו מִפְּנֵי חֲנֻכָּה,
וְעַל אֲדָר מִפְּנֵי הַפּוּרִים.
וּכְשֶׁהָיָה בֵית הַמִּקְדָּשׁ קַיָּם,
יוֹצְאִין אַף עַל אִיָּר מִפְּנֵי פֶסַח קָטָן.