Shivah Asar BeTammuz

The 17th day of the Hebrew month of Tammuz is a minor Jewish fast day. It commemorates the breach of Jerusalem's walls in 70 CE that led to the destruction of the Second Temple three weeks later. The day marks the onset of a period of three weeks of mourning practices, culminating with the fast day of Tisha B'Av, the day on which the Temple was destroyed.