A two-day Yom Tov in Chutz La’Aretz [any regular Yom Tov]: Rav: An egg laid on the first day is permitted on the second day. Rav Asi: It is forbidden.

Rabbi Zeira: Rav Asi’s opinion appeals to our logic. Because nowadays we know when Yom Tov really is but still keep two days [because we look at them as one long day].

Abayye: Rav’s opinion appeals to our logic. Because we learned in a Mishna: “In early times they would let everyone know when Rosh Chodesh was by means of signal fires. Later the Kusim began making false alarms and shluchim were sent out.”
However, if there wouldn’t be Kusim, we would go back to one day. Additionally, in those places [even outside of Eretz Yisroel] where shluchim could arrive in time, they would keep one day. [So this seems to be a proof to Rav that two days are only kept because of a safek and they’re not considered one Kedusha].

Challenge: If so why do we keep two days of Yom Tov now that we know the true date?

Resolution: An order was sent from Eretz Yisroel “Be careful to maintain the practice of your forefathers [to keep two days], lest the government interfere and we will fall back [since there were no calendars printed]”.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 [in the morning it is not a problem, since the witnesses usually come after the first Shir, but here the afternoon Shir was spoiled. Tosfos asks that they missed out on Mussaf also? They answer that the Mussaf can be brought after the Tamid b’dieved]. 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]. 

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