Halakhah zu Jirmejahu 26:78

The ninth day of Av (is a fast day), because on that day the Almighty decreed that our forefathers in the wilderness, would not enter Eretz Yisrael, for then the spies had returned (from exploring Canaan), and Yisrael cried in vain, and it was established as a day of weeping for future generations. On that day, the first Beis Hamikdash, as well as the second were destroyed, the city of Betar was captured, which was a large city, in which an immense number of Jews lived. On that day, Turnus Rufus plowed the site of the Beis Hamikdash, and the surrounding area, and, thereby, the verse was fulfilled, "Zion shall be plowed into a field" (Jeremiah 26:18). (There is another public fast day, the fast of Esther, see Chapter 141:2 below.)