The Fast of Gedalia (or Gedaliah) is a Jewish fast from dawn till dusk to commemorate the death of a Jew of that name.

When the Babylonian king Nebuchadnezzar conquered Jerusalem, he deported the poor inhabitants and left a simple man, Gedaliah, son of Achikam, in charge of the now-Babylonian province. Many Jews who had fled to Moab, Ammon, Edom, and other neighboring lands returned to the land of Judah, tended the vineyards given to them by the king of Babylonia, and enjoyed a new respite after their earlier oppression.

The King of Ammon however - hostile and envious of the Judean remnant - sent a loyal Jew, Yishmael Ben Netaniah, to assassinate Gedaliah. In the seventh month (Tishri), Yishmael came to Gedaliah in the town of Mitzpa, and was received cordially. Gedaliah had been warned of his guest's murderous intent, but refused to believe his informants, having the belief that their report was mere slander. Yishmael murdered Gedaliah, together with most of the Jews who had joined him and many Babylonians whom the Babylonian King had left with Gedaliah. The remaining Jews feared the vengeance of the Babylonian King (seeing as his chosen ruler, Gedalia, had been killed by a Jew) and fled to Egypt.

The surviving remnant of Jews was thus dispersed and the land remained desolate. In remembrance of these tribulations, Jewish sages instituted the 'Fast of the Seventh' on the day of Gedaliah's assassination in the seventh month.

There is some suggestion that Gedaliah was slain on the first day of Tishri, but the fast was postponed till after Rosh Hashanah, since fasting is prohibited during a festival. Concerning this fast day, the Rabbis have said that its aim is to establish that the death of the righteous is likened to the burning of the house of God. Just as they ordained a fast upon the destruction of the Jewish Temple, likewise they ordained a fast upon the death of Gedaliah.

When Rosh Hashanah falls on Thursday and Friday, the fast is postponed till Sunday, since no public fast may be observed on Shabbat (Saturday) with the exception of Yom Kippur.

The fast is observed from daybreak until the stars appear at night. The cantor includes the prayer Anenu in the repetition of the Shachrit amidah. A Torah scroll is taken (from the ark) the Thirteen Divine Attributes are said and the Passages of Vayechal are read from the Torah (Exodus 32:14 and 34:1-10).