The Seventeenth of Tammuz is a Jewish fast day commemorating the breach of the walls of Jerusalem before the destruction of the Second Temple. It falls on the 17th day of the Hebrew month of Tammuz and marks the beginning of the three-week mourning period leading up to Tisha B'Av. Like other minor fasts, Tzom Tammuz begins at dawn (first light) and ends at nightfall (full dark).

From hebcal.com.