Ta'anit Esther ("The Fast of Esther") commemorates the three days that Esther fasted before approaching King Ahasuerus on behalf of the Jewish people. The origninal fast that Ester made was on the 14th, 15th, and 16th days of Nisan(the eve and first two days of Passover). They fasted on Passover because Esther reasoned it would be better to fast on one Pesach lest they all be destroyed and thus never be able to observe the holiday in the future. The 13th of Adar was a fast day for the warriors while going out to battle, as it is believed to have been customary to fast before doing battle in order to gain divine favor. Because fasting during Passover would be inappropriate in almost all circumstances, the "Fast of Esther" became attached to the eve of Purim, the 13th of Adar. The fast is connected with Purim. If the 13th of Adar falls on a Friday or Saturday, it is moved to the preceding Thursday, because it cannot be moved forward a day (because it would fall on Purim).