Published January 14, 2010
There are 385 different Jewish Calendar dates - 353 of them occur every year, the 30th of Kislev occurs 74.5% of the time, the 30th of Marcheshvan occurs 44.9% of the time, and the 30 dates of Adar Alef occur 37% of the time.
Of the 385 dates, 295 of them are solidly linked together and follow the LO ADU ROSH pattern (or one of its 6 variations). These are the days from the first of Adar (either the first of two or the only one) all the way through and around the year until the 29th of Marcheshvan. These 295 dates are not affected by the variables in our calendar (namely, 30 Marcheshvan, 30 Kislev, and Adar Alef).
The 30th of Marcheshvan is the most “restricted” of dates and can fall only on SUN, TUE, or THU.
1-29 Kislev have only one day of the week (D/W) each on which they cannot fall.
The 30th of Kislev is also LO ADU, but it isn’t linked to the main set of dates.
59 dates (1-29 Tevet and 1-30 Shvat) each have two D/W on which they cannot fall. Rosh Chodesh Shvat, for example, cannot fall on a Sunday or Friday. The same is so for 2,9,16,23 Tevet and 8,15,22,29 Shvat. The other dates within this group of 59 form 6 other sub-groups (of 8 or 9 members each), each sub-group with its own 2 days on which they cannot fall.
* Knowing the details of the Jewish Calendar is part of the spirit of the very first mitzva given to Bnei Yisrael while we were still in Mitzrayim.
Even more so, USING the Jewish Calendar is part of this mitzva - why not start dating your checks with the Jewish date (in Israel, it is perfectly acceptable; don’t try this in Chutz LaAretz - come on Aliya instead!). One should know his or her Hebrew birthday and celebrate it (in addition to the “general calendar” date.)
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