The fast of the seventeenth day in the Hebrew month of Tammuz, known as Shivah Asar B’Tammuz, marks the beginning of a three-week mourning period for the destruction of Jerusalem and the two Holy Temples.
The day commemorates the breach of the walls of Jerusalem, the shattering of the Tablets of Law, and other historical calamities that came upon the Jewish people on this date.
Wishing all those fasting a meaningful and easy fast.

Source: Chanania Herman, Government Press Office