Regarding the two days of Rosh Hashanah, we learned that Rav and Shmuel agree that an egg laid on the first day is prohibited on the second day.
It was explained that this is because after the witnesses came late once, causing a confusion in the Shir, testimony was no longer accepted past Mincha. Thus if witnesses were to come late again, two days of Yom Tov would be kept while only the second day would be pronounced Yom Tov; thus lending a single extended Kedusha to both days. Rabbah then explained that from the time of Rabban Yochanan ben Zakkai, when it was no longer necessary to pronounce the second day Yom Tov on the first day, since he allowed accepting testimony all day following the destruction of the Beis Hamikdash, the law of the egg changed and it would be permitted on the second day.
Rav Yosef argued that nothing changed in the time of Rabban Yochanan ben Zakkai since the law prohibiting the egg is considered a davar Sheb’minyan which requires an official decision to permit it and the decision of Rabban Yochanan ben Zakkai’s Sanhedrin was only made regarding testimony on the first day, not regarding the egg.
Abayye: A decision was never made to prohibit the egg. The only decision ever made by the Sanhedrin was with regard to accepting testimony after Mincha. The status of the egg is contingent on that, but there was never any pronouncement made regarding the egg!