Tishah B'Av, observed on the 9th (tishah) of the Hebrew month of Av, is a day of mourning the destruction of both ancient Temples in Jerusalem. In modern times it has become a day to remember all Jewish suffering and loss.
I pleaded with the Eternal at that time, saying, "O Eternal God, You who let Your servant see the first works of Your greatness and Your mighty hand, You whose powerful deeds no god in heaven or on earth can equal! Let me, I pray, cross over and see the good land on the other side of the Jordan, that good hill country, and the Lebanon." - Deuteronomy 3:23-25