Bar Mitzva On Rosh Chodesh kislev This Year

This year 5785 it is not possible to become Bar mitzvah on Monday the second day of Rosh chodesh Kislev. Because 13 years ago there was only one day rosh chodesh kislev and this year there are 2 days rosh chodesh Kiev

There was a PSA from the Dirshu org.

 The משנה ברורה סימן נ"ה ס"ק מ"ה

If  a boy was born in a year that there was one day Rosh chodesh kislev as was the case 13 years ago and on the year of his bar mitzva there are two days rosh chodesh kislev as it is this year he must begin to put on תפילין on the first day of ראש חודש  December 1st although technically it is ל' חשון and still in the month of חשון

The Elyah Rabbah explains that unlike a yahrtzeit, the bar mitzvah does not go  by anniversary. Rather, it is timed according to when 13 years have passed. 

The Elyah Rabba, based on a ruling of the Bach maintains that if a boy is born on the 1st of Kislev in a year when Cheshvan had only 29 days, and in his Bar Mitzvah year Cheshvan has 30 days, then the boy becomes Bar Mitzvah on the first day of Rosh Chodesh Kislev, which is actually the 30th of Cheshvan! Since he completes 13 full years on that day (as there now is an extra day added to that year), he is obligated in Mitzvos on the day prior to his birthday! His birthday might be Alef Kislev, but his Bar Mitzvah is Lamed Cheshvan.

Although the Birur Halacha opines that it might still be preferable not to count this boy for a minyan until his actual birthday, and the rule would not apply for yahrtzeits nevertheless, this obscure ruling of becoming a Bar Mitzvah before the birthday is actually cited as halacha by the consensus of many great Acharonim, including the Shaarei Teshuva, Chavas Daas, Rav Akiva Eiger, Pri Megadim, Machatzis HaShekel, Levushei Srad, Mishna Berura and Kaf Hachaim

The Bobover Rebbe zt”l claimed he became Bar Mitzvah one day before his Bar Mitzvah date. He was born on Alef Kislev in a year when Cheshvan was chaseir (5668), and in his Bar Mitzvah year (5681) Cheshvan was malei.

This fascinating, albeit relatively unknown, psak would apply to this upcoming Rosh Chodesh Kislev as well. For, 13 years ago (5772), Cheshvan had only 29 days and this year (5785) it has the full 30 days. Therefore, a young lad who was born on the 1st of Kislev almost 13 years ago, and who is now on the cusp of manhood, would actually become Bar Mitzvah on the first day of Rosh Chodesh Kislev, which is actually the 30th of Cheshvan 5785, and not on his actual birthday, the second day of Rosh Chodesh Kislev, which is the 1st of Kislev.

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