Kewl Calendar Trivia

January 23, 2013

Ya know how people are always asking when the Jewish holidays will be for a given year …

“So, when is Passover/Rosh Hashanah/Hanukkah/ this year?”

To those unfamiliar with the Jewish calendar, the holidays seem to migrate from year to year. Well, I suppose they do  when your frame of reference is the Gregorian calendar. The reality is that Passover begins in the middle of Nissan; Rosh Hashanah falls on the Hebrew calendar dates of 1 and 2 Tishrei; and Chanukkah is always Kislev 25. Oh, yes, there are many more holidays on the calendar; this is but a mere sampling.

Okay, so the holidays have fixed dates, but, ‘why?’ you ask, do they move around on the real calendar? The short answer – the Hebrew/Jewish calendar is a lunar calendar consisting of 11 months; 12 in leap year.

So here is the kewl trivia I promised (with thanks to Asher Rappaport for sharing it on Facebook via Jonathan Mizrahi):

Hanukkah and Thanksgiving: A Once in Eternity Overlap

Next (Jewish) year features an anomaly for American Jews – The first day of Hanukkah coincides with Thanksgiving,  11/28/2013. It turns out that it has never rarely happened before…and it will [almost] never happen again.

Thanksgiving is set as the 4th Thursday in November, meaning the latest it can be is 11/28; 11/28 is also the earliest Hanukkah can be. The Jewish calendar repeats on a 19 year cycle, and Thanksgiving repeats on a 7 year cycle. You would therefore expect them to coincide roughly every 133 years (simple math 19 x 7). Looking back, this is approximately correct – the last time it would have happened is 1861. However, Thanksgiving was only formally established by President Lincoln in 1863. So, it has never happened before and, for all intents and purposes, will never happen again. Why, you ask …

…because the Jewish calendar is very slowly getting out of sync with the solar calendar, at a rate of 4 days per 1000 years (not bad for a many centuries old calendar!) This means that while presently Hanukkah can be as early as 11/28, over the years the calendar will drift forward, such that the earliest Hanukkah can be is 11/29. The next time Hanukkah falls on 11/28 will be in 2146 (which happens to be a Monday). Therefore, 2013 is the only year in which Hanukkah will overlap with Thanksgiving.

Of course, if the Jewish calendar is never modified in any way, then it will slowly move forward through the Gregorian calendar, until it loops all the way back to where it is now. So, Hanukkah will again fall on Thursday, 11/28…in the year 79,811 C.E.

For the true geeks & nerds, Mizrahi recently added an addendum filled with additional explanatory factoids.

Additional credit for this information can be tracked back to:

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