There are years during which there are two months of Adar, i.e., Adar I and Adar II. These years are known as leap years: years in which there are 13 months and not 12 months, as is the case in a usual year. Regarding a child born in the month of Adar of a regular year, and whose Bar Mitzvah takes place in a leap year, when would we celebrate that young man’s Bar Mitzvah — during בְּשמחָה Adar I or Adar II? The answer is: Adar II! Likewise, the mitzvah of reading Megillat Esther and all the mitzvot of Purim day are performed during Adar II. However, if a boy was born in Adar I during a leap year, and the year of his Bar Mitzvah is also a leap year, that boy would become Bar Mitzvah during Adar I.
