Q: Dear Rabbi Simon,
If someone was born (or died) on 30 Adar I, when would his birthday (or yahrtzeit) be in a normal year? (Adar usually only has 29 days.)
A: Dear Esther,
Thank you for your question. As you say, in a non-leap year (only one Adar) the month has only 29 days, so the question of marking an event which took place on 30 Adar 1 is an intriguing one.
Rav Yaakov Ettlinger (rebbe of Rav Samson Raphael Hirsch) in Binyan Tzion (1:151) says the boy celebrates his bar mitzvah on 30 Shevat, since that day is known as Rosh Hodesh Adar. However there are those who say he must wait until Rosh Hodesh Nissan. Interestingly, Rav Shlomo Zalman Auerbach is of the view that he should observe the stringencies of both statuses—minor and adult—during the month of Adar of his 12th/13th year. But in my opinion one who chooses to rely on the Binyan Tzion will not go far amiss.
With regard to yahrtzeit, it may be slightly different. Although the Mishna Berurah (568:42) quotes the Magen Avraham as saying the yahrtzeit is observed on 30 Shevat=RH Adar, Rav Moshe Feinstein (YD 3:159) writes that the correct text of the Magen Avraham is uncertain, and it may make more sense to observe the yahrtzeit on the first of Nissan, since the man was alive the previous year during all of Adar.
It is interesting that so much uncertainty surrounds this question which, while not very common statistically, must nevertheless have come up thousands of times in the course of Jewish history. After all, neither the maternity wards nor the funeral parlours are closed on 30 Adar 1, even if the day only comes around 7 times in 19 years.
So, to sum up: In my view, the boy is an adult on Rosh Hodesh Adar and the yahrtzeit is observed on Rosh Hodesh Nissan.