The discussion on yesterday’s daf was whether a beitza she-noldah be-Yom Tov (an egg that was laid on the holiday) was considered muktze on the second day of the holiday in the Diaspora. Our discussion focused on why we still keep a second day even at a time when we work with a set calendar and no longer need to communicate the establishment of the new month to far-flung communities.
The Gemara on our daf teaches that, on Rosh HaShana, all are in agreement that an egg laid the first day cannot be used on the second day, either. The Gemara points to a Mishna in Rosh HaShana (30b) as the source for this rule. The Mishna there told of an incident that took place in Jerusalem, where the witnesses who came to testify about the beginning of the month of Tishrei arrived in the late afternoon. By the time the Sanhedrin accepted their statement and announced that that day was Rosh HaShana, the service in the Temple had already begun and the Levi’im erred in the mizmor that they had begun singing. From that time on, two days of Rosh HaShana became normative, even in Israel.
Rabba comments that following the destruction of the Beit HaMikdash, the concern about mistakes in the Temple service no longer existed, pointing out that Rabban Yohanan ben Zakkai once again accepted testimony about Rosh Hodesh Tishrei all day.
Abaye said to him: But didn’t Rav and Shmuel both say that an egg [on the second day of Rosh HaShana] is prohibited? Rabba said to him: Your question is out of place; I say to you a statement in the name of the distinguished Rabban Yohanan ben Zakkai, and you say to me a ruling of the’im Rav and Shmuel? The Gemara asks: And according to the opinion of Rav and Shmuel, isn’t it true that the mishna is difficult, as it indicates that the special status of Rosh HaShana has been revoked? The Gemara answers that this is not difficult: This ruling is for us, those who live outside of, who have kept the ancient custom of observing two Festival days, and therefore Rosh HaShana is still considered one long day and constitute a single sanctity. Conversely, that ruling of the mishna is for them, the inhabitants of.
The simple reading of this Gemara seems to imply that, in Israel, only one day of Rosh HaShana is celebrated (see Rashi, who clearly understands the Gemara this way). The Ge’onim of Babylon, as well as the Ri”f and others, argue that Rabban Yohanan ben Zakkai did not change the basic rule, and even during his time Rosh HaShana was kept as a two day holiday. Nevertheless, other rishonim, including the Ramban and Rabbenu Efra’im, rule that in Israel only one day is kept, not only in the immediate vicinity of the Sanhedrin that establishes the new month, but in all of Israel, since we now rely on a set calendar.
There is historical evidence which seems to indicate that Rosh HaShana was kept for only one day in Israel until immigrants from Provence came and changed the tradition. Today the accepted practice is to celebrate Rosh HaShana for two days in Israel as well as in the Diaspora.