“ Megillat Tannit, they also nullified the prohibition of fasting before and after these holidays. (This nullification of the prohibition of fasting before and after a holiday mentioned in Megilat Taanit, did not include the exception of Hanukkah and Purim. Therefore, it is also permitted to fast the day before and the day after them.)
Although some argue (concerning whether one may fast the day before and after) Hanukkah, this is not the case concerning the Fast of Esther, which was universally accepted throughout the Jewish People. ”