Jews around the world fast from sunrise to sunset on the 13th of Adar. This fast day, called the Fast of Esther, is held just before the holiday of Purim. It is considered a minor fast and is more lenient in its observance, particularly when it comes to pregnant women, nursing mothers and others who are weak.
Though the fast is not mentioned in the Torah or Prophets (nor is it mentioned in the Talmud), there is a fast that is part of the Purim narrative and appears in the Book of Esther. Before appearing unbidden before King Ahasuerus, Esther fasted for three days and asked that all the Jews fast as well (Esther 4:16).
It is interesting to note that as a result of the fast, Jews hear the reading of the Book of Esther in the evening while still fasting. The Torah instructs soldiers to fast before going out to war. While reading the Book of Esther, the Jews of today are fasting, just like the characters in the story who fought the enemies of Israel in the days of Esther and Mordechai.
According to Jewish tradition, the three-day “Fast of Esther” mentioned in chapter 4 of the Book of Esther occurred on the 14th, 15th, and 16th days of Nisan, these being the eve and first two days of Passover. While Torah law normally forbids fasting on Passover, it is believed that Esther reasoned that it would be permitted to fast on Passover that year, lest they all be destroyed and thus never be able to observe the holiday in the future. But due to the normal prohibition of fasting on Passover, the “Fast of Esther” instead became attached to the eve of Purim, the 13th of Adar.
While the fast is generally celebrated on the day before Purim, when Purim is on Sunday, the fast is moved from Shabbat to the preceding Thursday.
The 14th century Bible scholar Rabbi Menachem ben Solomon Meiri taught that the Fast of Esther is a fast of joy and is not included in the group of the other public fast days which mark sorrowful events.