The 17th of Tammuz occurs exactly three weeks before Tisha B'Av, and begins a three week period of mourning for the destruction of the Beit Hamikdash. We fast because of five things that occurred on this day: The First Tablets were broken, the daily offering ceased, the walls of Jerusalem were breached, the evil Apostumus burnt a Torah scroll and an idol was set up in the Sanctuary.
