In the Jewish calendar, it is the end of the year, the twelfth month, and corresponds to March in the current solar calendar.
Esther 3:7In the twelfth year of King Ahasuerus, in the first month, which is the month of Nisan, they cast Pur (that is, the lot) before Haman for each day and each month, and the lot fell on the twelfth month, which is the month of Adar.
Esther 9:21to establish that they should keep the fourteenth and fifteenth days of the month of Adar annually,