Fasting Before and After Purim
The Fast of Esther, celebrated before Purim, on the 13th of Adar, is not an original part of the latter, nor was it later instituted "in commemoration of the fasting of Esther, Mordecai, and the people", since this fasting fell, according to rabbinical tradition, in the month of Nisan and lasted three days. The first who mentions it is R. Aḥa of Shabḥa (8th cent.) in "She'eltot," iv.; and the reason there given for its institution is based on an arbitrary interpretation of Esth. ix. 18 and Meg. 2a, "The 13th was the time of gathering," which gathering is explained to have had also the purpose of public prayer and fasting. Some, however, used to fast three days in commemoration of the fasting of Esther; but as fasting was prohibited during the month of Nisan the first and second Mondays and the Thursday following Purim were chosen. The fast on the 13th is still commonly observed; but when that date falls on a Sabbath the fast is put back to Thursday, Friday being needed to prepare for the Sabbath and the following Purim festival.
Purim Katan
In leap-years on the Hebrew calendar, Purim is celebrated in the second month of Adar, but by the Karaites in the first. The respective days of the first Adar being then called "Purim Katan" (Little Purim), for which there have been set forth certain observances similar to those for Purim proper, with the exception of reading the Megillah, sending gifts to the poor, and fasting on the 13th of the month. The distinctions between the first and the second Purim in leap-years are mentioned in the Mishnah (Meg. i. 46b; comp. Orah Hayyim, 697).
See also: Jewish holidays