The fast is meant to commemorate the death of Gedaliah, a righteous Jew who was the governor of the land of Judah following the collapse of the First Temple at the hands of the Babylonian Empire. However, he was assassinated by his fellow Jews, specifically by Ishmael Ben Nethaniah, who descended from the Davidic line.Dr.Tzvi Novick, Tzom Gedaliah: Why Commemorate His Assassination? (TheTorah.com)
His death was recounted in detail by the Roman-era historian Josephus, and is also described in the book of 2 Kings chapter 25 and, in even more detail, in the Book of Jeremiah chapter 41.
Gedaliah ben Ahikam, the governor of Judah after the destruction of the Temple, was assassinated by Ishmael ben Nethaniah, a scion of the Davidic family. This event has been commemorated for millennia with a yearly fast—the only fast over the death of an individual. The Talmud points to his righteousness, while Saadia Gaon emphasizes the tragic consequences to the Judahite people he governed.
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