Months in the Jewish calendar consist of either 29 or 30 days. When a month continues for 30 days, two days of Rosh Hodesh are observed – on the 30th day of the month, and on the first day of the next month.
Most months are consistent, having the same number of days each year. There are, however, a small number of months that fluctuate, as they sometimes have 29 days and sometimes 30. One such month is Heshvan.
If a boy was born on the 30th of Heshvan in a year when it had 30 days, and Heshvan in the year of his Bar-Misva has only 29 days, then he becomes a Bar-Misva on the first day of Kislev. One might have thought that we define this boy's birthday as the day prior to the first of Kislev, such that he becomes a Bar-Misva on the 29th of Heshvan. But this is not the case. We instead view him as having been born on Rosh Hodesh Kislev, and so he becomes Bar-Misva on the only day of Rosh Hodesh Kislev in the year of his Bar-Misva – namely, the first of Kislev.
The Poskim dispute the Halacha in the reverse case, when a boy was born on Rosh Hodesh Kislev in a year when Heshvan had only 29 days, and there are two days of Rosh Hodesh Kislev in the year of his Bar-Misva. The Misha Berura and the Kaf Ha'haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) approvingly cite the view of the Bayit Hadash (Rav Yoel Sirkis, Poland, 1561-1640) that the boy in this case becomes a Bar-Misva on 30 Heshvan, the first day of Rosh Hodesh. According to this opinion, we view the boy as having been born on the day after the 29th of Heshvan, and thus in a year when Heshvan extends for 30 days, his birthday is the 30th of Heshvan.
Rav Meir Mazuz (1945-2025), however, disagreed. In his view, since the boy was born on the first of Kislev, this date is his birthday, and so he becomes Bar-Misva on this date irrespective of how many days the previous month had. Even if there are two days of Rosh Hodesh in the year of his Bar-Misva, he does not become a Bar-Misva until the date of his birth – the first of Kislev. Hacham David Yosef, in Halacha Berura, ruled that the boy in this case should observe the stringencies of both opinions – meaning, he should ensure to observe all the Misvot already on the 30th of Heshvan, but should not be counted toward a Minyan until the first of Kislev.
Summary: If a boy was born on the 30th of Heshvan in a year when Heshvan had 30 days, and Heshvan in the year of his Bar-Misva has only 29 days, he becomes a Bar-Misva on the first of Kislev. If he was born on Rosh Hodesh Kislev in a year when Heshvan had only 29 days, and there was thus only one day of Rosh Hodesh, but Heshvan in the year of his Bar-Misva has 30 days, it is unclear when he becomes a Bar-Misva. Therefore, he should ensure to observe all the Misvot already on the 30th of Heshvan, but should not be counted toward a Minyan until the first of Kislev.