The origin of the fast of Esther is actually another fast in Jewish history that comes from the inter-testamental period. That day was known as Yom Nikanor, or the Fast of Nikanor. Nikanor was the Greek Syrian general who was defeated by Judah Maccabee in the year 160 BC. For some reason, the Jews began observing a fast on the day that he was killed, and the fast day was the thirteenth day of Adar, or the twelfth month. Later, this fast was transferred to Esther’s fast. Esther actually fasted, according to Jewish tradition, in the month of Nisan, but it was later forbidden to fast in Nisan, so the fast was transferred to the month of Adar. The date of the fast is the thirteenth of Adar, which is the day before the feast of Purim. There are three reasons for the fast.
The first reason is that it is based upon Esther 4:16, which records the three days of fasting by Esther. Originally, the fast was kept consecutively for three days. Later it was not kept consecutively, but kept on the Monday, Thursday, and Monday preceding the feast of Purim. Still later, it became only a one-day fast, the day preceding the feast itself.
The second reason for the fast was to remember the Holy One who sees and hears the prayer of every person in time of distress when they fast and return to God with all their heart, with all their soul and with all their strength (Deuteronomy 6:4-9).
The third reason for the fast was that it was a day that the enemies of the Jews were killed, so it should be viewed as a day of mourning, not a day of feasting.
Lastly, there are two rabbinic laws concerning the fast of Esther. First, the fast is not obligatory as the other four fast days of Scripture and so this one may be relaxed in special cases such as: that of a pregnant or nursing woman; those who suffer from eye problems, or a groom who is within seven days of his wedding need not fast. Secondly, if Purim falls on a Sunday, since one is not allowed to fast on either Friday or Saturday by Jewish tradition, the fast is observed on the preceding Thursday.128