Related for Rosh Hashanah 1:3
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:
For six months messengers go out. [When beth-din sanctified the New Moon, they sent out (messengers) to inform the exile which day they had sanctified, whether the thirtieth, so that the month past was defective; or the thirty-first, so that the month past was full. (And the messengers of beth-din may desecrate neither Shabbath nor yom tov.)]: For Nissan, as it affects (the date of) Pesach; for Av, as it affects the fast (of the ninth of Av) [more afflictions having befallen us then than on (the dates of) the other fasts.]; for Elul, as it affects Rosh Hashanah [They apprise us when it is Elul, and Rosh Hashanah is observed on the thirtieth of Elul in the exile. For on most years Elul is not intercalated. And even though there is the possibility that beth-din might intercalate it, they (those in the exile) have no way of knowing this, so that, perforce, they must follow the majority of the years. And if they did not know when Elul began, they would not know its thirtieth day.]; for Tishrei, as it affects proper institution of the festivals. [The messengers go out the day after beth-din sanctified Tishrei, and go as far as they can until the festival, apprising them whether or not beth-din had intercalated Elul, so that they not be apprehensive on Yom Kippur and Succoth (as to whether they are observing them in their proper times)]; for Kislev, as it affects Channukah; and for Adar, as it affects Purim. And when the Temple stood, they would also go out for Iyyar, as it affects "the minor Pesach" [Pesach Sheni].