Which Jewish fast occurs the day after Rosh Hashanah, unless delayed by Shabbat?

The Jewish fast that occurs after Rosh Hashanah, unless delayed by Shabbat, is the Fast of Gedalia.

It commemorates the assassination of Gedaliah son of Ahikam, the Babylonian-appointed governor of the surviving Jewish community in Judah after the destruction of the First Temple. The murder is described in 2 Kings 25 and in greater detail in Jeremiah 41. His death ended the remaining Jewish autonomy in the land and prompted many survivors to flee to Egypt.

The fast is observed on 3 Tishrei, which is normally the day after the second day of Rosh Hashanah. If 3 Tishrei falls on Shabbat, the fast moves to 4 Tishrei, because public fasting is generally not observed on Shabbat. It lasts from dawn until nightfall and is one of Judaism’s minor fasts.

A common mix-up is assuming that the fast marks a universally agreed date of the assassination. The Bible does not specify the day, while rabbinic tradition places it on 3 Tishrei; other interpretations connect the event with the first of the month and explain the later fast date as a postponement.