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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?