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This site explains that there are 4 rules that would cause Rosh Hashannah to be postponed 1 or 2 days after the date of Molad Tishrei. You can view all 4 rules on the web site, but rule 4 states:

4) If molad Tishri following a leap year falls on Monday at 15 hr 589 hq (9:32:43 1/3 AM) or later, Tishri 1 is delayed by one day for the following reason:

Molad Tishri of the leap year occurred on or after Tuesday at 18hr (noon) Therefore Tishri 1 of that leap year was delayed one day by (2 - Molad Zaken rule) and one more day by (1 - Lo Ad"u Rosh rule) This would make that leap year too short (382 days) Note that this delay would now cause Tishri 1 to fall on a Tuesday and that will never cause (1) to trigger a further delay

I have designed an Excel sheet with some Visual Basic programming to list the dates of Rosh Hashannah and tell me the postponement rules (if any) that were applied. I have listed all years from 5775 until 5900. I have not been able to locate any instance where this rule has been applied. Granted, I may have a logic error in my program, so perhaps, it exists after all.

Would anyone know when this rule was last applied and when the next occurrence would occur? Also, if you know of a quick way to determine this programmatically (VNA or psudeu code), I'd really appreciate that.

While I'm at it - Shana Tovah to all.

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  • Just as a rough estimate, the rule only applies if it falls between 15.589 and 18 hours which is 2651 Chalakim. There are 181440 Chalakim in a week, so the Molad should occur at the right time about 1.5% of the time. But leap years are only 7/19, so the Molad at the right time after a leap year is only 0.5% of the time, or once every 200 years. You only checked 125 years, so you shouldn't have been surprised to have missed it.
    – Double AA ♦
    May 16, 2016 at 19:45

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It has been used 11 times so far since the enactment of the fixed calendar in 4119. It was used in 4179, 4257, 4504, 4602, 4849, 5096, 5194, 5441, 5519, 5688, and 5766. The next time it will be used is 6013 if we still have a fixed calendar then. You can read about this exceedingly rare Dechiya and its history here.

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  • Thanks for the research and the link. I'll have to brush up on my Ivrit a bit :-) Shanah Tovah
    – DanF
    Sep 23, 2014 at 3:23
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    +1 - the date of the enactment of the calendar is known with such precision?
    – Y     e     z
    Sep 23, 2014 at 3:25
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    @YEZ Apparently it's based on some Teshuva from the Geonim. I wouldn't take it as an Ikkar Emunah.
    – Double AA ♦
    Sep 23, 2014 at 5:34
  • @Yez hakirah.org/Vol20Ajdler.pdf
    – Double AA ♦
    Oct 19, 2016 at 2:00
  • Years of Betutakpat 75 153 322 400 647 745 992 1239 1584 1662 1831 1909 2078 2156 2501 2748 3093 3340 3587 3685 3932 4010 4179 4257 4504 4602 4849 5096 5194 5441 5519 5688 5766 6013 6111 6358 6605 6950
    – Double AA ♦
    Jun 10, 2020 at 19:42

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