The Seventeenth of Tammuz (Hebrew: שבעה עשר בתמוז‎, Shiv'ah Asar b'Tammuz) is a minor Jewish fast day commemorating the breach of the walls of Jerusalem before the destruction of the Second Temple.^[1] 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.^[2]

The day also commemorates the destruction of the Twin Tablets of the Ten Commandments. The Seventeenth of Tammuz occurs forty days following the holiday of Shavuot. Moses ascended Mount Sinai and remained up there those forty days. The Children of Israel built the Golden Calf on the afternoon of the sixteenth of Tammuz when it seemed that Moses was not coming down when promised. Moses descended the next day (forty days by his count), saw that the Israelites were violating many of the laws he had received, and smashed the tablets.^[3]