(Mishnah): For six months, the messengers go out - in Nisan because of Pesach, in Av because of the fast (9th of Av), in Elul because of Rosh Hashanah, in Tishrei to ensure the correct timing of the festivals, in Kislev because of Chanukah, in Adar because of Purim. And when the Temple stood, they would also go out in Iyar because of Pesach Sheni.

(Gemara -question): Why did they not go out for Shavuos?

Answer: Shavuos always occurs at a fixed time (the day after the 49th day of the Omer).

This shows that Shavuos can occur on the 5th, 6th or 7th day of Sivan. If both Nisan and Iyar are full months, Shavuos will occur on the 5th. If one is full and one is not, it will occur on the 6th. If neither are full months, it will occur on the 7th.

Question: Did the messenger not already go out for Elul? Why should they again go out for Tishrei?

Answer: In order to inform the people on which day Rosh Hashanah had occurred (depending on whether or not Elul was a full month).

R. Yehoshua ben Levi: I can guarantee that those people who go to Nimurin will not die before their time, because the custom in that place is to keep Yom Kippur for two days (due to the doubt about the day).

R. Chisda to those people: Why do you put yourselves into this doubt? We can assume that Beis Din will not be lazy in the matter (and it is dangerous to fast for two days).

The father of R. Shmuel bar Rav Yitschak was stringent upon himself to fast for two days. His intestines became detached and he died.