The fast begins at 5:15am and ends at 6:09pm (NYC Times)
Because Adar 13 (the day before Purim) falls on a Shabbat this year, the “Fast of Esther,” usually observed on that date, is moved back to today.
Haman’s “Final Solution” was scheduled for the 13th of Adar. Instead, Haman was hanged, and by royal decree the Jews mobilized in self-defense and successfully defended themselves against all attacks. On the next day, the 14th of Adar, the Jews rested, savored their miraculous salvation, and established the holiday of Purim.
Whenever in peril, we are commanded to institute a fast day to repent, pray, and ask for Divine mercy. Thus the 13th of Adar, the day of battle, was a day of fasting.