While the Hebrew calendar is based mainly on the lunar cycle, it includes a mechanism designed to keep the calendar in tandem with the solar year, which is approximately eleven days longer than twelve lunar months. Therefore, every two or three years an extra month is inserted into the year, forming a “leap year,” shana me’uberet in Hebrew. This extra month is added at the end of the year, after Adar, and is called “Second Adar.” The periodic leap year ensures that the month of Nisan, containing Passover, will always fall during the spring, in accordance with the commandment of the Torah: “Observe the month of ripening” (Deuteronomy 16:1), a reference to spring.

As explained above, in a leap year there are two months of Adar. The second Adar is the main Adar, and most of the laws and customs of Adar apply then. Purim is celebrated in Adar II, and if a person was born in the month of Adar in a non-leap year, he will mark his birthday in Adar II in a leap year.

Adar I is also a special month, and the statement that rejoicing increases during the month of Adar applies to that month as well. In addition, the fourteenth and fifteenth of the first Adar are called Purim Katan (“Minor Purim”) and Shushan Purim Katan, respectively. On these days, due to the joy associated with them, one does not recite Tahanun.

For information that is useful when determining the date of a bar mitzva, or a yahrzeit for a death that occurred in a leap year, see p. 37 and p. 120.