There are five minor fasts on the Jewish calendar. With one exception, these fasts were instituted by the Sages to commemorate some national tragedies. The minor fasts (that is, all fasts except Yom Kippur and Tisha b'Av) last from dawn to nightfall. There is a great deal of leniency practiced in these minor fasts for people who have medical conditions or other difficulties fasting. The date of the fast is moved to Sunday if the specified date falls on a Sabbath.
Three of these five fasts commemorate events leading to the downfall of the first commonwealth and the destruction of the first Temple, which is commemorated by the major fast of Tisha B'Av (which is a required fast, unlike these minor fasts).
Following is a list of minor fasts customarily thought to be required by Jewish law, their dates, and the events they commemorate:
The Fast of Gedaliah, Tishri 3, commemorates the killing of the Jewish governor of Israel, a critical event in the downfall of the first commonwealth.
The Fast of Tevet, Tevet 10, is the beginning of the siege of Jerusalem. It has also been proclaimed a memorial day for the six million Jews who died in the Holocaust.
The Fast of Esther, Adar 13, commemorates the three days that Esther fasted before approaching King Ahasuerus on behalf of the Jewish people. The fast is connected with Purim. If Adar 13 falls on a Friday or Shabbath, it is moved to the preceding Thursday, because it cannot be moved forward a day (it would fall on Purim).
The Fast of the Firstborn, Nisan 14, is a fast observed only by firstborn males, commemorating the idea that they were saved from the plague of the firstborn in Egypt. It is observed on the day preceding Passover.
The Fast of Tammuz, Tammuz 17, is the date when the walls of Jerusalem were breached.
While these fasts are permitted, they are not required according to Mishneh Torah (and the so-called Fast of the Firstborn is not even mentioned, as it is such a new custom). Those who wish to voluntarily practice these fasts (other than the Fast of the Firstborn), which are an honored and ancient tradition, are not engaging in forbidden ascetic practices, even though they are not strictly required.
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