TSK · Esther 9:21

In the first month(that is, the month of Nisan), in the twelfth year of King Ahasuerus’ reign, pur(that is, the lot) was cast before Haman in order to determine a day and a month. It turned out to be the twelfth month(that is, the month of Adar).

The Origins of the Feast of Purim But the Jews who were in Susa assembled on the thirteenth and fourteenth days, and rested on the fifteenth, making it a day for banqueting and happiness.