The Fast of Esther
The fast that precedes the holiday of Purim.
The Fast of Esther is post-Talmudic in origin. In this article the author provides a number of possible reasons regarding the development of this fast day. The reason for the wide range of possibilities is that there is no clear explanation accounting for the late emergence of Ta'anit Esther. Excerpted with permission from Celebrate! The Complete Jewish Holiday Handbook (Jason Aronson Inc.).
A day of fasting from sunrise to sunset is supposed to be observed on the day before Purim (Adar 13). It ostensibly commemorates the fast Mordecai and Esther endured, which Esther instituted among all the Jews, prior to her visit to the king (Esther 4:16). In keeping with Judaism's system of measure-for-measure reward and punishment, it is the flip side of feasting in celebration of Purim's outcome: denying pleasure to the body appropriately atones for the transgressions committed by the Jews when they shamelessly indulged their bodies during King Ahasuerus' banquet.
Fasting was also commonly practiced among Jews whenever they prepared for battle (as with the Persians who had been instructed to massacre them), in remembrance that their strength and victory would come from God.
The fast on Adar13 became the custom well after other observances were adopted for Purim, possibly as an adaptation of the periodic Monday and Thursday fasts the Jews followed. While it carries less obligation than the fasts ordained in Tanakh[the Hebrew Bible] and others in Talmud, some, particularly the Persian (Iranian) Jews, have kept it as faithfully, reciting special selihot (penitential) prayers and selections of Torah recited on all other fast days. [The same is true for traditional Jews outside of Iran as well.]