Daf Yomi - Rosh Hashana 18 - Informing the Public

The Mishna states that at the beginning of six months, messengers would be sent out to notify the Jewish communities as to which day was determined to be the first of the month. They were sent out in the month of Nissan because of the festival of Pesach. At the beginning of Av, they were sent out because of the fast. Messengers were sent in the month of Elul on the account of Rosh Hashanah. They were sent out during Tishrei because of Yom Kippur and Sukkos. Messengers were sent out during Kislev because of Chanukah and during Adar because of Purim.

Pnei Yehoshua questions the necessity of the messengers at all according to the opinion of those Tanaim who maintain that there is always a four day difference between the day that the festival falls out in one year and the day that the festival occurs in the following year. Since it is known which month will be thirty days and which month will be twenty-nine days, what is the purpose of the messengers? He answers that even according to that viewpoint, there are times that Beis Din will be forced to deviate from the regular cycle and change the amount of days in the month due to the seasons or because of a leap year. It is therefore still necessary to send out the messengers in the beginning of every month.

Reb Leib Mallin answers this question differently with a novel approach. There is a halacha based on pesukim in the Torah that Rosh Chodesh can only be established through Beis Din. There is a secondary halacha, based on that, which teaches us that informing the public when Rosh Chodesh was also must be accomplished through Beis Din . He learns this from the language of the Ranbam in Hilchos Kiddush Hachodesh (1:7). It emerges that even if it is known precisely when Rosh Chodesh was, there is nevertheless an obligation for Beis Din to notify the public.