צום גדליה
The fast of the seventh month is צום גדליה. It is commemorating a tragedy in Jewish History that coincidentally took place in the beginning of the month of Tishrei.
The incident took place after the destruction of the First Temple by the Babylonians. Shortly after this tragic event, Nevuchadnezzar made Gedalia Ben Achikam, the governor over a handful of Jews that were allowed to return to Jerusalem. Many Jews who had fled to Moav, Ammon, and Edom, returned to tend their vineyards.
The King of Ammon was angered by this small remnant of the Jewish people. He sent Yishmael Ben Netaniah, a Jew, to assassinate Gedalia.
Despite being tipped off of this plot, Gedalia refused to believe such a thing was possible. Some say the ultimate assassination of Gedalia and many other Jews, took place on the first of Tishrei, but the fast was postponed to the third.
After this horrible event, the Jews dispersed. This ended any hope of the rebuilding of the Beit Hamikdash. The Rabbis decreed that this was to be an annual fast day on the Jewish calendar.