The chances of being born on February 29 are (very close to) 1 in 1,461. That makes leap-year babies somewhat rare. When do people born on February 29 celebrate their birthday, especially in non-leap years? The strict Februarians celebrate on February 28. Others celebrate on March 1. According to Wikipedia, in the United Kingdom those born before noon on February 29 have their birth certificate dated February 28; those born after noon are dated March 1. The other option for those “leapers” is just to celebrate their birthday once every four years. So on their fifth birthday, they are already able to drive a car.
While the above discussion may be somewhat academic, the ramifications of a Jewish leap year are profound. When should a boy who was born in Adar during a leap year celebrate his bar mitzvah, if his bar mitzvah year is not a leap year? When should a boy born during a non-leap year celebrate his bar mitzvah, if his bar mitzvah is a leap year? Adar Rishon or Adar Sheini? Should we just give the bar mitzvah boy his choice? Does it depend on whether the halls are booked? When will the boy be considered an adult for the myriad of halachos that depend on one’s adult status?
According to the Beis Yosef, these answers can be found in a Gemara that we just learned in Arachin (31b). The halachah is that one should preferably give his firstborn domesticated animal (bechor) to a kohein before its first birthday. Suppose there were two sheep born during a leap year: one on the 15th of Adar Rishon, and one on the 1st of Adar Sheini. Which animal would be one year old first in the following non-leap year? Rebbe Abba bar Mamal ruled that the animal that was born second, on the first of Adar Sheini, will turn one year old on the first of Adar. The animal born first will turn one two weeks later, on the 15th of Adar.
The Beis Yosef concludes that the same halachah applies to bar mitzvah boys. A boy born during a leap year, whether in Adar Rishon or Adar Sheini, whose bar mitzvah year is not a leap year, will celebrate his bar mitzvah on the same date in Adar. For example, a boy born on the tenth of Adar Rishon will become bar mitzvah on the tenth of Adar. A boy born on the tenth of Adar Sheini will also become bar mitzvah on the very same day, the tenth of Adar. This can lead to an intriguing situation: One boy is born on the 29th day of Adar Rishon; another boy is born two days later, on the 1st day of Adar Sheini. If their bar mitzvah year is not a leap year, then the boy who was born first will become bar mitzvah on the 29th of Adar, while the boy who was born two days later will become bar mitzvah much earlier, on the 1st of Adar!
We can even take this one step further. Suppose a boy is born on the 30th of Adar Rishon. When does he become bar mitzvah during a non-leap year? A non-leap-year Adar has only 29 days. The Mahari Mintz rules that he becomes bar mitzvah on the first of Nisan. Now suppose that he had a twin brother that was born ten minutes later than him and after sunset. This twin was born on the first of Adar Sheini. He will become bar mitzvah during a non-leap year on the first of Adar. His twin brother, who was born first, will have to wait a full month later, until the first of Nisan, to become bar mitzvah!
The Shevus Yaakov raises the following question: In the above scenario, which brother is considered older? There are at least two potential ramifications of this question. The first is that there is a halachah that a younger brother should honor his older brother. The second is in regard to the mitzvah of yibum. If a man dies childless, there is a mitzvah for the man’s brother to marry his wife. (This is no longer practiced; chalitzah is performed instead.) The halachah is that it is a mitzvah for the oldest brother to perform this rite. In the scenario above, which brother is considered older: the one that was born first or the one that turns bar mitzvah first? The Shevus Yakkov said that we consider the brother that was born first to be the older brother. So while the “younger” brother is celebrating his bar mitzvah, he will still have to honor his “older” 12-year-old kid brother.
What about the reverse situation? A boy is born on the 5th of Adar during a non-leap year, but his bar mitzvah occurs in a leap year. When does he become bar mitzvah—on the 5th of Adar Rishon or the 5th of Adar Sheini? The Rema (O.C. 55:10) writes that he becomes bar mitzvah on the 5th of Adar Sheini. In other words, on the 5th of Adar Sheini he may be counted towards a minyan. However, Rav Sholom Eliezer Roter wrote in the name of Rav Elyashiv, shlita, that one must consider the boy to be bar mitzvah on the 5th of Adar Rishon for all stringencies. If this boy’s family had a tradition to put on tefillin starting from his bar mitzvah (that is, even if he wouldn’t normally start putting on tefillin beforehand), he must start on the 5th of Adar Rishon. However, he will not be counted towards a minyan until the 5th of Adar Sheini. This is because there is some element of doubt as to the correct month that he is actually bar mitzvah in. So we are machmir both ways.
What is the halachah if a boy is born on the 15th of Adar Rishon and his bar mitzvah is during a leap year as well? The answer would seem to be obvious: the 15th of Adar Rishon. The Magen Avraham disagrees. His reasoning is as follows: This boy turned 12 on the 15th of Adar during a non-leap year. Another year must pass until he becomes bar mitzvah. We consider a year to have passed on the 15th of Adar Sheini. Consequently, on the 15th of Adar Sheini this boy will turn 13. As a proof, he offers the halachah regarding the sale of a house in a walled city that is also referred to in our Gemara in Arachin.
The Torah tells us that if a man sells a house in a walled city in Eretz Yisrael, he may force the buyer to sell it back to him within one year of the original sale date. If the original sale date was the 15th of Adar, when is the first anniversary of the sale date if the following year is a leap year? According to the Chachamim quoted in a mishnah in Arachin, the answer is the 15th of Adar Sheini. We can conclude that a year starting on the 15th of Adar terminates on the 15th of Adar Sheini. This boy as well has to wait a year from the day he turned 12 until he turns bar mitzvah.
However, many Acharonim disagree with the Magen Avraham. The Mishnah Berurah rules that a boy born on the 15th of Adar Rishon will in fact be bar mitzvah on the 15th of Adar Rishon, not like the Magen Avraham.
If you find this subject confusing, try reading it after a few drinks at your Purim seudah.

Rabbi Sebrow leads a daf yomi chaburah at Eitz Chayim of Dogwood Park in West Hempstead. He can be contacted at ASebrow@gmail.com.