The Fast of Esther (Ta'anit Ester, Hebrew: תַּעֲנִית אֶסְתֵּר) is a fast on Purim eve commemorating two communal fasts undertaken by the Persian Jewish community of Shushan in the Book of Esther, for the purpose of praying for salvation from annihilation by an evil decree which had been instigated by Haman, the king's royal vizier, an anti-jewish enemy from the Amalekite nation.
[1] Esther asked Mordechai to have the Jews fast and pray on her behalf for 3 days and 3 nights, before she approached her husband, king Ahasuerus to beg for the life of her people.
Another is presumed to have occurred on the 13th of Adar, the day before the Jews fought a battle to defend against their enemies, who had been given an irrevocable permission to murder Jews for one day a year on the 14th of that month, which is now a Jewish holiday of Purim on account of their victory.
[4] A 2010 study examines the origin of the fast and the reason for its arising in the Gaonic period.
(Shulchan Aruch S.686 s.2) As the fast of Esther is not one of the four public fasts ordained by the Prophets, the laws concerning its observance are more lenient; pregnant women, nursing mothers, and those who are weak are not required to observe it.