Mishneh Torah, Sanctification of the New Month 4:2 (Sefaria English translation with my modification on the word "Aviv"):

עַל שְׁלֹשָׁה סִימָנִין מְעַבְּרִין אֶת הַשָּׁנָה. עַל הַתְּקוּפָה וְעַל הָאָבִיב וְעַל פֵּרוֹת הָאִילָן

For three indicators we make the year a leap [year]. For the epoch and for the barley crop ripening and for the fruit of the trees.

Currently, our fixed calendar is concerned only with the 1st reason as the other two applied only during Temple times. A Judaic leap year occurs every 2 or 3 years, according to our current "fixed" calendar system.

My understanding of this idea is that we want to make sure that 16 Nissan occurs in the Spring season, i.e. after the Spring Equinox, which is March 20 or 21.

A Judaic non-leap year ranges between 353 and 355 days. Thus, each Judaic non-leap year, Pesach will be between approx. 10 to 12 days earlier on the solar calendar than the previous year.

Last year, Pesach was on April 4, 2015. Had this year not been a Judaic leap year, Pesach would have occurred at its earliest (assuming a 354 day year) on March 24 (subtracting 12 days would have placed Pesach on a Monday which is not possible, so it would have occurred on Tues. March 24.) This still after the Spring Equinox.

As a matter of fact, it seems that whenever there is a 2-year interval, had that leap year not been there, Pesach would still have been after the Spring equinox. (See 5784 / Greg. 2024 as another example.)

Why, then are leap years sometimes placed at the 2-year interval, when, seemingly, it may not be necessary?

I haven't looked at the full possibility that a leap year could possibly occur exactly every 3rd year. I have to experiment with that. However, my main question is why we don't make a leap year only when it's really needed, rather than a year before it's needed. I suspect that there some efficiency in doing it the way we have it?

  • IAE as usual the answer drift due to imprecision in the calendar judaism.stackexchange.com/a/68710/759
    – Double AA
    Apr 4, 2016 at 21:55
  • You couldn't do it every third year. You'd have to push one of the two-year intervals off to being a three-year interval and then resume the regular schedule after that.
    – Daniel
    Apr 4, 2016 at 23:08

1 Answer 1


If the leap year occurred every third year, the drift would accumulate too much. For ease of calculation use 354 days per "normal" lunar year and 384 days per leap year. This means that there would be 1,092 days in three years while there are 1,095.75 solar days in three years. This would mean that the residual drift would be a full extra month every 30 years.

Our current calendar (of seven leap years in a 19 year cycle give 6936 days per cycle which compares to 365.25 x 19 = 6,939.75 days in 19 solar cycles. Note that I used only the mean calculation. The extra days even this out even more.

Part of the reason that we use a fixed calendar is because we do not have a court with the authority to change the calendar. Once the fixed calendar was set up, we would require a court that was greater than the original court to make any changes.

It should be noted that even with the fixed calendar set up as it is, there is still a slow drift forward in the calendar. Part of this is the difference between the Julian and Gregorian calculation of not having a leap year in 3 out of 4 century years. This is also the reason that the time to start saying V'Sein Tal U'Matar has drifted forward to December fifth over the centuries between the setting up of the calendar and today. Since the fixed calendar was set up, the dates of Pesach had drifted and will eventually have to be dealt with. However we cannot change the calendar until we actually have a Sanhedrin with the authority to make any needed changes.

It should be noted, that since Pesach must be the fifteenth of Nisan, we cannot define the occurrance of Pesach as the "first full moon after the Solar equinox". We had to define the calendar so that Rosh Chodesh Nisan (on the lunar calendar) occurs as close (before) that point as possible. Once the calendar was set up, we no longer had the authority to skip a leap year or change the spacing one time to account for the millenial drift.

I deal with the matter on my blog at What will happen to the calendar if the Sanhedrin is re-instituted

  • This doesn't answer the question
    – Daniel
    Apr 5, 2016 at 12:18
  • @Daniel The question that I got from the post was Why, then are leap years sometimes placed at the 2-year interval, when, seemingly, it may not be necessary? and what would be the effect of every three years without change. That is what I answered. I point to the link in which I point out how the Sanhedrin might react once it is reconstituted. I also point out that we do not have a court with authority to declare a leap year outside the fixed calendar that we have. Apr 5, 2016 at 12:22
  • I understand the question to be asking why the fixed calendar is set up this way, not why we use the fixed calendar at all. The OP is proposing that the calendar could have been set up such that Pesach always falls on the first full moon after the equinox
    – Daniel
    Apr 5, 2016 at 12:26
  • @Daniel I added some more explanation. Apr 5, 2016 at 12:40
  • @Daniel and sabbahillel. I need to absorb and digest the above answer, as it does provide useful info. My main question is along your lines of thinking, Daniel. I'm mainly curious why the leap year is not placed only when it's needed, i.e., only if Pesach would occur before the spring equinox. Yes, I get the idea that if we postponed this year to next, eventually there would be a 2 year interval somewhere else. Fine. But, we would still have the leap years just when we needed them, not "earlier". I also have to view the linked question, Daniel. You may have answered it, there.
    – DanF
    Apr 5, 2016 at 13:27

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