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Rosh Hashana is the only festival celebrated for two days even in Jerusalem, since there was a possibility that witnesses for the new moon would come in the middle of the day and the Beis Din would declare it Rosh Hashana (retroactively). If the witnesses didn't come, Rosh Hashana would (biblically) automatically be deferred to the next day, but Melacha would continue being prohibited due to the fear that people would Mezalzel the first day of Rosh Hashana the next year.

Nowadays, Rosh Hashana always falls out on the 1st and 2nd. So the calendar is programmed "as if" the witnesses came the first day.

Why do we have a second day?

  • Nowadays Sukkot falls on 15 and 16. Why do we have two days? – Double AA Oct 7 '14 at 0:59
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    (Btw before 900 years ago they didn't keep two days RH in Israel. machonshilo.org/en/images/stories/files/…) – Double AA Oct 7 '14 at 1:19
  • @DoubleAA That's sfeika de'yoma. No Sfeika DeYoma here – Shmuel Brin Oct 7 '14 at 2:24
  • Shmuel There's no Sfeika DeYoma there either. We know it's the 15th. We keep two days because sometimes they kept two days. Same with Rosh HaShana. – Double AA Oct 7 '14 at 2:38
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It was actually the subject of a machlokes Rishonim if Rosh HaShana should be two days in Jerusalem/Beis HaVaad/Israel. The Baal Me'or (Beitza 3a) cites part of the discussion, and records that the practice was to only keep one day, until it was changed per the psak of the Rif.

The Rif claims that based on Rava (that R' Yochanan would agree even after the enactment of Raban Yochanan Ben Zakai to return acceptance of testimony to the whole day, that an egg born on day 1 of Rosh Hashana is still forbidden on day 2) that even in Israel Rosh Hashana should be 2 days.

Rabbeinu Efraim (cited by Rosh and Baal Me'or) says that once the calendar was established, all of Israel is like the Beis HaVaad where there is no doubt and they keep one day.

The Rashba and the Rosh defend the Rif. The Rashba says that from the fact that outside of Israel didn't change with the establishment of the calendar, we see that the establishment of the calendar didn't change these things. The Rosh says a slightly different variation - that from the fact that outside of Israel they still forbid an egg born on day 1 of Rosh Hashana to be eaten on day 2 of Rosh Hashana, we see that even after the establishment of the calendar it is still treated as one long day.

The Ramban says a different proof - that all that needs to be proven is if the 2 days of Rosh HaShana were an enactment or just a reaction to a doubt. Once it is proven that they are an enactment, they do not revert just because the doubt is resolved.

  • The rest of Israel could also have Sefeika Deyoma (They couldn't reach Teverya within a day, even if they were allowed to violate Techum). – Shmuel Brin Oct 7 '14 at 2:25
  • @ShmuelBrin It doesn't actually matter where there was or wasn't sefeika deyoma. We don't have a ring of places that keep two days Sukkot but not two days Shmini Atzeret. – Double AA Oct 7 '14 at 2:38

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