10. [In a leap year,] on the fourteenth and the fifteenth of the first
month of Adar (1), the "Tachanun" prayers and the passages "E-l Erech
apayim" and "Lam'natzeach" should not be recited. It is forbidden to
recite eulogies ("hesped") or to fast on these days, and it is proper to
eat a slightly larger meal ("marbim ke'tzas be'seuda") on the fourteenth.
(1) A lunar year is approximately 11 days shorter than the solar year;
therefore, in order to keep the Jewish festivals (which are based on the
lunar cycle) in their correct seasons (for example, Pesach in the spring),
an extra month of Adar is added every few years (the Jewish calendar works
in cycles of 19 years with years 3,6,9,11,14,17 serving as leap years).
During a leap year, we celebrate Purim on the 14th and 15th of the second
month of Adar, however, as today's halacha states, we still treat the 14th
and 15th of the first Adar as days of increased joy.