Rav Yosef: 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].
Another Proof: [At the time of Matan Torah, there was a strict prohibition against ascending the Mountain. We would think that after Matan Torah it would be self understood that this no longer applies, but instead] it is written “When the Shofar sounds, they can ascend the Mountain”.
Another Proof from a Mishna: Fruits of a fourth year vine must be brought up to Yerushalayim from a radius of a day’s journey in every direction [people who live near Yerushalayim cannot redeem it and bring money]. This is the boundary: Eilas in the South, Akravas in the North, Lud in the West and Yarden in the East.
Ulla or perhaps it was Rabbah bar bar Chana quoting Rabbi Yochanan: The reason for this is so that the markets of Yerushalayim will be crowned with fruits.
Beraisa: Rabbi Eliezer had a fourth year vine East of Lud [within the above boundary] near Kfar Tavi. He desired to gtive away all the fruits to poor people so that he would be free of the burden of taking it to Yerushalayim [this was after the churban, when there was no longer any reason to crown the markets of the conquerors, yet he still felt bound by this law]. His disciples said to him: Rebbe your Chaverim already permitted it.
The Chaverim refer to Rabban Yochanan ben Zakkai.
We see that Rabbi Eliezer needed to hear that there was special permission even though the reason seemingly no longer applied.
Question: Why were more proofs necessary?
Response: The first verse was not enough because perhaps the command to return to the tents was not necessary to permit the prohibition. It was just because the men were so enamored by Matan Torah that they would have forgotten about Mitzvas Onah [they would forget about the big function of the Jewish people to make children] - thus the second verse is necessary.
However these two proofs only discuss D’Orayso laws, so the Mishna about a fourth year vineyard was necessary to teach that this law applies in D’Rabbonon cases as well.
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 sometimes two days of Yom Tov were kept even when only the second day was 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, 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.
Rav Yosef: If you want to argue that Rabban Yochanan ben Zakkai permitted the egg as well, this is incorrect since the decision of his Sanhedrin was only made regarding testimony on the first day, not regarding the egg.