The Fast of Esther (Ta'anit Ester, Hebrew: תַּעֲנִית אֶסְתֵּר‎) is a Jewish fast from dawn until dusk on Purim eve, commemorating the three-day fast observed by the Jewish people in the story of Purim. It is a common misconception that this fast was accepted by the Jews for all future generations during the time of Esther, as it is stated in the Book of Esther: They had established for themselves and their descendants the matters of the fasts and their cry (Esther 9:31). This verse actually refers to the four fasts which relate to mourning for the Temple. Rather, the first mention of this fast is a Minhag that is referenced in the Gaonic period.^[1] However, the fast itself is considered a law, that developed under the authority of the last legitimate Sanhedrin.

The Fast is observed on the 13th day of the Hebrew month of Adar. If the date of the Fast of Esther falls on Shabbat (Saturday), the fast is instead observed on the preceding Thursday, as is the case in 2010, 2011, 2013 and 2014.^{[citation needed]}