This date is associated with the breach of the walls of Jerusalem by the Romans in 70 AD and it initiates the three-week period of mourning leading up to Tisha b'Av. Prior to the fall of the second temple, the Fast of Tammuz was associated with the fall of the first temple and is referenced in the Book of Zechariah, Zech 8:19, specifically: "And it came to pass, the word of the Lord of Hosts to me, saying, 'Thus sayd the Lord of Hosts: A fast of the 4th [month]... will become for the House of Judah for happiness and for joy, and for good appointed times; and the truth and the peace they will love.'" Traditionally, the Fast of Tammuz is also associated with Moses' smashing the two tablets containing the 10 Commandments, when he saw the idolatry of the Hebrews around the golden calf.
Tisha B'Av, Hebrew for the 9th day in the month of Av is, the anniversary of the destruction of the first and second temples in Jerusalem (586 BCE by the Babylonians, and 70 AD by the Romans, respectively). This date is associated with other crimes against the Jews, such as the expulsion of the Jews from Spain in 1492. In synagogues, the Book of Lamentations is read on Tisha b'Av. This is a fast day for observant Jews. Traditionally, there no weddings and no B'nai Mitzvah between the Fast of Tammuz (17 Tammuz/June 25, 2013) and Tisha b'Av.