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There are some debates about the details of calendar calculations among earlier Poskim about how to calculate the calendar, especially around pushing Rosh Hashana off deliberately.

For example, Hebrew Wikipedia records a divergence between R. Saadia Gaon and Ben Meir that cause a divergence for the years 4683 and 4684.

Did this type of argument ever cause different communities to actually have a different calendar, and thus have some holidays on a different day, for a given year, or were the disagreements only argued but never implemented? (I'm curious about the results on the ground of the 4683/4684 controversy as well: did that argument ever lead to actual divergent practices?)

  • If yes, I'd be interested to learn if those calendars slowly diverge from each other over time such that as the years go on, the average amount of time between the same "date" in each calendar increases. – Daniel Aug 4 '16 at 18:49
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    @Daniel That is unlikely. Rosh Hashana is determined by the molad that year. Any difference in one year (such as a one day push off -- Dechiya) would be balanced quickly by Cheshvan/Kislev +/- 1 so the next RH would work out right. Unless someone disagrees about the length of a month 29/12/793. – Double AA Aug 4 '16 at 18:50
  • @DoubleAA but if they have two different ways on how to calculate the molad, then you can have two communities calculating the molad two different ways and end up with two different days of Yom Tov, no? – Mennyg Aug 4 '16 at 21:26
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    @Mennyg That's what I said at the end about disagreeing about the length of the month. A molad is just by definition a month past the previous molad. – Double AA Aug 4 '16 at 21:27
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That "example" -- Ben Meir and Israel vs R' Saadia Gaon, the Gaonate, and Babylonia is the only such dispute over the calendar.

I "published" on Avodah a copy of an article by R' Yosef Gavriel Bechhofer and R/Dr Ari Z Zevetofsky at Avodah v5n35 #8: Calendar Controversy Article. They propose an interesting possibility for the cause of the dispute.

You ask about what people did in practice. This actually threatened to split rabbinic Judaism, as Jews in Israel were following Ben Meir over R' Saadia Gaon. Realize that this is before Talmud Bavli became "The Gemara", at least in the eyes of the community whose ancestors were taught by the amoraim of Israel, as recorded in the Talmud Yerushalmi. It was also at a time when Qaraism was new and upsurgant.

Thus the heat of the battle was more over the unification of rabbinic Judaism under the gemara than the actual calendar issue itself. This trend then continues into Ashkenaz, where much Eretz Yisrael practice co-existed with Babylonian rulings. (At least according to the dominant theory in the history of halakhah.) And now that the the gemara and geonim gained universal acceptance they had to find an understanding of halakhah that includes both the babylonian texts and practices that come from a wider variety of sources. And by the end of a couple centuries, the challenges yield the schools of Tosafists.

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