On the Origin of Species September 29, 2022 Q I cultivate aravos and other shrubs…
Month to Month
Dayan Yosef Greenwald
November 4, 2021
Q The yahrzeit of my father a”h is the 30th of Cheshvan, but this year Cheshvan has only 29 days. On which day do I observe the yahrzeit?
A A lunar cycle, a single orbit of the moon around Earth, takes 29 days, 12 hours, and 793 1080ths (chalakim) of an hour, or 44 minutes and 3-1/3 seconds. Because this figure is between 29 and 30 days, and a single day cannot be part of two months, all months have either 29 or 30 days. Since the establishment of our current calendar nearly 17 centuries ago, the months are fixed in duration, with the exception of Cheshvan and Kislev, each of which is 29 days in some years and 30 in others.
The Magein Avraham rules that in your case, where the passing was on 30 Cheshvan, the timing of the yahrzeit depends on the structure of the calendar in the year of the first yahrzeit. 30 Cheshvan is both the last day of Cheshvan and the first day of Rosh Chodesh Kislev. If the first yahrzeit falls in a year with a 29-day Cheshvan, the 29th, being the last day of Cheshvan, is the yahrzeit, and in all subsequent years, the last day of Cheshvan—be it the 29th or the 30th—is the yahrzeit. But if the first yahrzeit is in a year with a 30-day Cheshvan (which means there are two days of Rosh Chodesh Kislev, 30 Cheshvan and 1 Kislev), the yahrzeit is permanently fixed as Rosh Chodesh Kislev, and the yahrzeit will always be either 30 Cheshvan or 1 Kislev, depending on the length of Cheshvan.
The Sha’arei Teshuvah disagrees, but the minhag follows the Magein Avraham. As a result, two people who each lost a parent on 30 Cheshvan can have different yahrzeits, depending on the length of Cheshvan in the year of each one’s first yahrzeit.