Tisha b'Av

Tisha b’Av refers to the 9th day of the month of Av on the Jewish calendar. The significance of Tisha b’Av is that it commemorates a number of disasters in Jewish history, including the destruction of both the First and Second Jerusalem Temples (in 586 BC and AD 70, respectively).

Tisha b’Av: Remembering the Loss of Zion
http://www.hebrew4christians.com/Holidays/Summer_Holidays/Tishah_B_Av/tishah_b_av.html

The 9th of Av – Tisha b’Av: A Day of Mourning, A Day of Hope
http://www.chabad.org/library/article_cdo/aid/144575/jewish/The-9th-of-Av-Tisha-BAv.htm