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How about the situation, common in the Diaspora, where we celebrate two days of Yom Tov, one after another? Can an egg laid on the first day of Yom Tov be used on the second day?
The Gemara teaches that this is the subject of a disagreement between Rav, who permits its use on the second day, and Rav Assi, who forbids it.
Rav Assi was uncertain whether the two days of Yom Tov in the Diaspora are deemed to be one continuous day of holiness or not, so he recited havdalah in case the two days of Yom Tov were not deemed to be one continuous day of holiness, and Rav Assi also ruled that an egg that is laid on the first day of Yom Tov is prohibited on the second day of Yom Tov.
Although we now know when Rosh Chodesh is established, the reason we still observe two days of Yom Tov is because a message was sent from Eretz Yisroel to Babylonia, “be careful to follow the custom of your fathers who observed two days of Yom Tov, because the gentile government may issue a decree that it is prohibited to study Torah,” and this will cause the Torah scholars to err in calculating the lunar cycle and this can lead to people eating chametz on Pesach.
We explore the concept of Yom Tov Sheni across the demninational spectrum especially those shuttling between Israel and the diaspora these days with such ease.