The dilemma of the date of the 30th of Adar 1 and how one can commemorate a birthday/Yartzite:

A. What is one to do if he was born on the 30th of Adar I and in the current year there is only one Adar of 29 days?[1]

If one was born on the 30th of Adar of a leap year and the current year is a non-leap year, which hence only contains 29 days in Adar, then some Poskim[2] rule one celebrates his [birthday and] Bar-Mitzvah on the 30th of Shevat. He is considered from that day and on like a complete Gadol. Other Poskim[3] however rule he is to celebrate on the first of Nissan. Practically, he is to celebrate on the 30th of Shevat and is considered Bar Mitzvah regarding all personal matters starting from this day, although it is proper to be stringent regarding fulfilling a Biblical obligation on behalf of others, to not do so until the 1st of Nissan.[4] Regarding the birthday celebration perhaps it is proper to celebrate the birthday customs on both dates, the 30th of Shevat and on the 1st of Nissan.[5]

B. When is one to commemorate a Yahrzeit of a relative who passed away on the 30th of Adar I and in the current year there is only one Adar of 29 days?[6]

If a person passed away on the 30th of Adar Rishon of a leap year and the current year is a non-leap year, which hence only contains 29 days in Adar, then he is to commemorate the Yahrzeit on the 30th of Shevat, which is the first day of Rosh Chodesh Adar.[7] However, there are Poskim[8] who rule that he is to commemorate it on the first of Nissan. [Practically, one is to commemorate the Yahrzeit on the 30th of Shevat, and if he desires to be stringent, also on the 1st of Nissan.[9]]

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[1] See Piskeiy Teshuvos 55:18; Nitei Gavriel 12:4 footnote 7

[2] Binyan Tziyon 141; Mishpatecha Leyaakov 13; Minchas Yitzchak 6:8; 8:7; Vayaan Yosef  33; Even Yisrael9:63; Vayan David 1:11; See Teshuvas Mahariy Mintz 9; Har tzevi 2:83; So also rules regarding a Yartzite: One version of M”A 568:20 [following the Girsa of the Machatzis Hashekel and Yad Efraim in M”A 568:20, however see P”M ibid and Levushei Serud for a different Girsa]; Mateh Efraim 3:8; Kitzur SHU”A 221:3; M”B 568:42; Kaf Hachaim 568:42; Nitei Gavriel 77:14

[3] Birur Halacha 55 in his conclusion and in name of Steipler; Emek Hateshuvah 2:5; Igros Moshe Y.D. 3:159 regarding a Yartzite; See Nitei Gavriel 12:4 footnote 7 and Piskeiy Tehsuvos ibid footnote 130 for the different opinions on this matter; So rules regarding one who was born on the 30^th of Cheshvan or Kisleiv that he does not become Bar Mitzvah until the 1^st of Kisleiv/Teves, and should seemingly be the ruling here as well: Admur 55:13; M”A 55:10; Elya Raba 55:9; Yad Ahron; M”B 55:45; Ketzos Hashulchan 15:6; Derech Hachaim; Kneses Hagedola; Poskim in Kaf Hachaim 55:60; So rules regarding one who was born on the 1^st of Kisleiv and the next year there are two days Rosh Chodesh Kisleiv, that he celebrates only on the 1^st day of Kisleiv: Shaareiy Teshuvah 55:5 in name of Elya Raba

[4] Piskeiy Teshuvos 55:18; Nitei Gavriel ibid; Heard from Harav Yekusiel Farkash

Regarding joining a Minyan: One may be lenient for him to join. [Piskeiy Teshuvos ibid and footnote 131 based on fact that some Poskim rule a Katan can join a Minyan and it is a merely Rabbinical matter] However, some are stringent. [See Vayaan David ibid; Piskeiy Teshuvos ibid]

[5] So answered the Rebbe in a similar scenario of one who was born on the 30^th of Cheshvan, that he should celebrate twice on the 29^th and the 1^st. [Heichal Menachem 2:38-9]

[6] See Piskeiy Teshuvos 568:11; 55:18; Nitei Gavriel 77:14

[7] One version of M”A 568:20 [following the Girsa of the Machatzis Hashekel and Yad Efraim in M”A ibid, however see P”M ibid and Levushei Serud for a different Girsa]; Mateh Efraim 3:8; Kitzur SHU”A 221:3; M”B 568:42; Kaf Hachaim 568:42; Nitei Gavriel 77:14; See also Minchas Yitzchak 6:8 who rules this way regarding a birthday

The reason: As Adar of a plain year is always in place of the second Adar of a leap year, and hence we view that the death occurred on the first day of Rosh Chodesh Adar of a plain year, which is the 30^th of Shevat. [M”A ibid, in accordance to above Girsa]

[8] Igros Moshe Y.D. 3:159 based on P”M ibid and Levushei Serud who have a different Girsa in M”A ibid; So rules regarding one who was born on the 30^th of Cheshvan or Kisleiv that he does not become Bar Mitzvah until the 1^st of Kisleiv/Teves: Admur 55:13; M”A 55:10; Elya Raba 55:9; Yad Ahron; M”B 55:45; Ketzos Hashulchan 15:6; Derech Hachaim; Kneses Hagedola; Poskim in Kaf Hachaim 55:60; and so rules regarding one who was born on the 1^st of Kisleiv and the next year there are two days Rosh Chodesh Kisleiv, that he celebrates only on the 1^st day of Kisleiv: Shaareiy Teshuvah 55:5 in name of Elya Raba

[9] So conclude regarding a Bar Mitzvah: Piskeiy Teshuvos 55:18