Misconceptions
It is generally accepted in the rabbinic tradition that the original three-day "Fast of Esther" mentioned in chapter 4 of Book of Esther occurred on the 14th, 15th, and 16th days of Nisan, these being 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 during the 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.
Read more about this topic: Fast Of Esther