The fast days of 17 Tammuz and 9 Av, and the time between them, commemorate the tragic process of the destruction of the two Holy Temples in Jerusalem.
The fast of the 17th of Tammuz, the day of the breaching of the walls of Jerusalem, marks the beginning of the Three Weeks". The fast of the 9th of Av, commemorates the day of the actual destruction by fire of the Holy Temple, and is the last day of the Three Weeks. The three weeks is a period of mourning over these destructions.

The Three Weeks, for the year 5768 / 2007-2008, begin on July 20, 2008 (17th of Tammuz), and continue through August 10, 2008 (9th of Av).