About 2,371 years ago, two days before Pessach, Haman decreed that all the Jews would be destroyed on 13th Adar, eleven months later.
After consulting with her cousin Mordechai, Queen Esther declared a 3-day fast that lasted through the first days of Pesach, after which she went – uninvited – to the king and invited him and Haman to a wine feast.
Ta’anis Esther is the only one of the 6 fast days not mentioned in Tanach.
The fast ends when the stars become visible (a few minutes before Motzai Shabbat time), and if needed, one may eat and drink before Megilla reading. However, one should not eat a meal until after hearing the Megilla.
Source: Kitzur Shulchan Aruch 141:2,8, Esther 2:7, 3:12-9:30
Tuesday, 12 Adar-II 5776