I was born on Rosh Hodesh Adar 1. During a leap year, should I celebrate my birthday on the first or the second Adar?
Since you were born in the first Adar in a leap year, your birthday then takes place in the first Adar (according to Mishnah Berurah 55, 43). You are not obligated to "celebrate" your birthday. My great grandfather used to say with humor that the only birthday which appears in the Torah is that of ... Pharaoh! “Now it came about on the third day, Pharaoh's birthday, that Pharaoh made a feast for all his servants" (Genesis 40, 20). This topic is brought down in Halachah in the context of indicating the day of a bar-mitzvah where a boy turns from a Katan to a Gadol who is obligated in all of the commandments. It says in Shulchan Aruch (Orach Chaim 55, 10), One youth may have been born on the twenty-ninth day of the first Adar of a leap year and another youth may have been born on the first day of the second Adar of the same year. In such a case, if the year in which they turn thirteen is not a leap year, the youth who was born on the twenty-ninth day of the first Adar will have to wait until the twenty-ninth day of Adar of the year in which they turn thirteen in order to be considered thirteen years of age. On the other hand, the youth who was born after him, on the first day of the second Adar, will be considered thirteen years of age already when the first day of Adar of the year in which they turn thirteen arrives. The Mishnah Berurah (43) adds that this law applies only if the year in which they turn thirteen is not a leap year, but if the thirteenth year is also a leap year, we obviously rule in such a case that the youth who was born in the first Adar will become bar-mitzvah in the first Adar and the second youth who was born in the second Adar will become bar-mitzvah in the second Adar. [This is agreed upon by the Achronim, in contradiction to the view of the Magen Avraham]. Rema: Someone who was born in Adar in a non-leap year and becomes bar-mitzvah in a leap year, will not become bar-mitzvah until the second Adar (Responsa Mahari Mintz 15). The Mishnah Berurah (45) quotes the Levush who explains why the youth born in Adar in a non-leap year does not become bar-mitzvah in the first Adar but in the second Adar: The first Adar is not considered an Adar, but is termed an intercalary month, as may be seen from the fact that the Megilah is not read in the first Adar. A youth born in Adar must therefore wait until the second Adar, since thirteen complete years are required before one can be classed as grown. Happy Birthday!