We Learned: Two days of Rosh Hashanah: Rav and Shmuel agree that an egg laid on the first day is forbidden on the second day. This is based on the Mishna: At first testimony regarding Rosh Chodesh was accepted all day. Once, the witnesses came late and the levi’im sang the wrong Shir as a result. From then on they no longer accepted testimony after Mincha. If witnesses did come later, they would keep the remainder of the day holy, and pronounce the next day Rosh Hashanah as well. [So many times, two days were kept even though they knew that only the second day would be pronounced Rosh hashanah].
Rabbah: From the time of Rabban Yochanan ben Zakai’s Takanah,an egg laid on the first day of Rosh Hashanah is permitted on the second day.
As we learn in the Mishna: After the churban, Rabban Yochanan ben Zakai made a takanah permitting the acceptance of testimony all day on Rosh Hashanah (since there was no longer a concern about interfering with the Shir).
Abayye to Rabbah: But Rav and Shmuel both agree that an egg is forbidden?
Rabbah: I’m telling you about Rabban Yochanan ben Zakai and you’re bringing up Rav and Shmuel?
Challenge: Rav and Shmuel appear to be challenged by the Mishna?
Resolution: The law is different for us and for them [in Eretz Yisroel there are no longer two days of Rosh Hashanah, but in Chutz La’Aretz we still keep two days following the old system established when the two days were considered one Kedusha (016)].
Rav Yosef: The egg remains forbidden even after Rabban Yochanan ben Zakai’s Takanah.
This is because it was a davar sheb’minyan [something taken up for vote in the Sanhedrin and decided by the majority], and something decided by a majority requires a majority decision to undo it.
Rav Yosef: I learn this principle from the verse “Return to your tents”. [At the time of Matan Torah they were told to separate from their wives, but after Matan Torah they had to be told to return, otherwise they wouldn’t be allowed to since it is considered like a davar sheb’minyan].