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When Rosh Chodesh is two days, it's on the 30th of the old month and the first of the new month. Rosh Hashana falls on the beginning of the new month of Tishrei. Why then is Rosh Hashana on Tishrei 1 and 2 instead of Elul 30 and Tishrei 1?

(I know there is no Elul 30; Elul always has 29 days. Yom Kippur is on Tishrei 10 which we count from the first day of Rosh Hashana, so we can't just call it Elul 30 and Tishrei 1 -- this affects all the other days that month.)

Rosh Hashana is the only yom tov that begins a month, and so I understand that halacha's treatment of it is different from the others: it's two days everywhere, including in Eretz Yisrael, and I've been told that the rabbis (in the talmud?) describe it as one long day. But none of that tells me why it's shifted a day from other months.

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Here's a piece on Chabad.org explaining it. Basically, it is on Elul 30 and Tishrei 1 -- the only problem is that Elul was set to only have 29 days.

Back in the days of witness-based Rosh Chodesh, Elul sometimes had 29 and sometimes had 30 days. If it ended up having 29, then the day after (Tishrei 1) would be Rosh Hashanah and Tishrei 2 would be an ordinary day.
However, if the moon wasn't sighted by the time Elul 30 came around, what to do? Perhaps it would be sighted later that day, and then the day would retroactively be RH. But, it might not be. It was decreed that the 30th of Elul would be treated as RH lechumra. If it turned out that it wasn't, the actual day would be observed on the first of Tishrei.

In the rest of the land, they wouldn't know when the day was established. After all, once it became RH, the court couldn't notify people far away. If i read it correctly, they would always observe 2 days because they wouldn't be sure.

Since even during the time when the calendar was based on the sighting of the new moon two days of RH were observed, it was decreed that it would still be like that even with a fixed calendar. And since Elul was set to have 29 days, the 2 days are Tishrei 1 and 2.

  • Hopefully this is comprehensible. I almost didn't post it, but figured that even if it's not a perfect answer, it should at least help get people thinking. – Scimonster Aug 31 '15 at 13:34
  • I don't think this answers the question. Why not set Elul to have 30 days and observe on 30 Elul and 1 Tishrei? – Daniel Aug 31 '15 at 13:34
  • @Daniel Why would you want to do that?? – Double AA Aug 31 '15 at 13:35
  • @DoubleAA Consistency. – Daniel Aug 31 '15 at 13:35
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    @Daniel ??? The current way is much much much more consistent. Imagine if Israelis kept only the first day of pesach but only the second day of sukkot. That would be weird as anything. I don't even know what consistency you are aiming for. – Double AA Aug 31 '15 at 13:36
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You understand, I think, why there was a doubt about the day of Rosh Hashana and they needed two days (on the day after 29 Elul no one knew if witnesses would come or not, etc.). Thus, in fact, every date (eg.) on the Hebrew calendar had a one-day doubt to it (if you lived far enough away that messengers couldn't get to you in that number of days), either forward or backward depending on which way the calendar 'defaults' to that month (ie if the previous month had 29 or 30 dates).

The question you should be asking is why don't we have two day Rosh Chodesh after every month because of a doubtful day, because as it is now no month but Tishrei has a two day Rosh Chodesh due to that reason. The answer will hopefully soon be available here: Why not two days of Rosh Hodesh, as we have two days of yontif?

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