Who Makes the Hebrew Calendar, Then? Originally Published July 26-27, 2019.

Last week we talked a bit about the rules that govern the creation of the Hebrew calendar, and someone asked who makes the calendar.  The answer is “We do!”

As we mentioned, there are rules that were already being followed and were codified in the sixteenth century in the Shulhan Arukh (Orakh Ḥayyim 428), including the days of the week on which certain holidays may not fall, etc., and including which parashah should be read when.  On the Shabbat before Pesaḥ, for example (we wrote), we must read Tzav, unless it is a leap year in which case it would be Metsora; however, if Rosh Hashanah was on a Thursday, it would be Aharei Mot

Well, this coming year on the Shabbat before Pesaḥ we will be reading Tzav, and the Shabbat before that we will read Vayak-hel + Pekudey.  That Shabbat is Shabbat HaHodesh, the last of four special Shabbatot before Pesaḥ.  It falls on the Shabbat before the month of Nisan begins, unless Nisan begins on Shabbat, in which case Shabbat HaHodesh coincides with Rosh Hodesh.  We have a young soon-to-become bat mitzvah studying this parashah!

The Jewish calendar is lunisolar, the months go according to the moon and the years according to the sun.  The mean synodic month is 29 days, 12 hours, 44 minutes, and 3⅓ seconds long.  (An hour is divided into 1,080 parts, each of which is 3⅓ seconds.)  The solar year is 365 days, 48 minutes, and 46 seconds, which means that the solar year exceeds a lunar year of 12 months by approximately 11 days.  In the 19 years of a lunar cycle, the solar cycle exceeds the lunar by about 209 days.  So an extra month is added in each of seven of the 19 years that constitute the lunar cycle, making each an “embolismic” year.  Also, “full months” (מָלֵא) have 30 days, and Nisan, Sivan, Av, Tishrei, Shevat, and Adar I (in leap year) are always “full.”  Iyyar, Tammuz, Elul, Tevet, Adar, and Adar II in leap year are always “short” חָסֵר 29 days.  The others vary.

Now, to start the year.  If Rosh Hashanah is calculated to fall on Sunday, Wednesday, or Friday, it is postponed by a day.  If the new moon appears after noon, Rosh Hashanah is postponed by a day, and if the new day is Sunday, Wednesday, or Friday, it is postponed another day.  In a regular 12-month year (not leap), if the Tishrei new  moon appears (over Jerusalem) more than 20 seconds past 3:11 a.m. on a Tuesday, Rosh Hashanah is postponed by two days.  In a year after a leap year, if the new moon appears on a Monday more than 43 seconds past 9:32 a.m., Rosh Hashanah is postponed by a day.  So how do we postpone it?  We add days to Heshvan and Kislev beforehand!

A friend remembers his dad calculating the calendar with charts and a slide rule.  What fun!  Okay, maybe now some gentle readers are saying that the answer is “someone else creates the calendar.”