Baal HaTurim: “The appointed festivals of HaShem” The word מעודי is spelled full with a Vav (Gematria of 6). This alludes to the Mishneh (Rosh haShana 1:3), “regarding six months, Bais Din sends out messengers.”
Rosh Chodesh (the first day of the month) is declared by the Bais Din based on the testimony of two valid witnesses who testify to actually seeing the molad (first appearance of the emerging crescent moons after its dark phrase) on the thirtieth night of the previous month. Bais Din then proclaims that day as Rosh Chodesh. If valid testimony is not accepted, then the thirty-first day is proclaimed as Rosh Chodesh.
Thus, it was necessary to inform the nation as quickly as possible so that they would know when to celebrate Yomim Tovim. Therefore, Bais Din would send out messengers throughout the Jewish world. However, they would only send out messengers in the six months which contained a Yom Tov: Nissan due to Pesach; Av because of the fast; Elul due to Rosh haShana; Tishrei for Yom Kippur and Sukkos; Kislev for Chanukah; Adar due to Purim. During the time of the Bais HaMikdash they would not go out for Av (since no fast) but went out for Iyar due to Pesach Sheni.
