Jewish Minor Fasts 101
In addition to the major public fast days of Yom Kippur (the Day of Atonement) and Tisha B'Av (the Ninth of the month of Av, which commemorates the destruction of the Temples), there are four minor public fast days in the Jewish calendar. The minor fasts last only from dawn until sundown, in distinction to Yom Kippur and Tisha B'Av, which last 25 hours, from sundown until three stars can be seen in the sky the following night. The minor fast days also are not as restrictive in practice as the major fasts.
Most of the minor fasts are connected with the destruction of the Temple. The Seventeenth of Tammuz (Shiva Asar be-Tammuz) commemorates the day on which the Romans breached the walls of Jerusalem in 70 C.E. It marks the beginning of the three weeks of mourning leading up to Tisha B'Av. The Fast of Gedaliah (Tsom Gedaliah) is in memory of the Babylonian appointed governor of Judah, who was assassinated by a fellow Judean following the destruction of the First Temple. It takes place on the day following Rosh Hashanah.
The Tenth of Tevet (Asarah be-Tevet) marks the beginning of the Babylonian siege of Jerusalem in 586 B.C.E. The Fast of Esther (Ta'anit Esther) is observed on the day before the joyous holiday of Purim, unless that day is a Sabbath. In fasting on this day, one is emulating the fast of Queen Esther and her fellow Jews before she went to the king to plead on their behalf.