Tisha B’Av (the ninth of Av) is a fast day to mourn terrible events in Jewish history. As referenced in Zechariah 7:3, the fifth month (Av) is a traditional time of fasting and mourning.
According to Mishnah Ta’anit 4:6, “Five misfortunes befell our fathers … on the ninth of Av. …On the ninth of Av it was decreed that our fathers should not enter the [Promised] Land, the Temple was destroyed the first and second time, Bethar was captured and the city [Jerusalem] was ploughed up.” This day is remembered by fasting and reading the book of Lamentations.
We know in the future, G-d says this day of mourning and others “will become joyful and glad occasions and happy festivals” according to Zechariah 8:19. When we celebrate Tisha B’Av, we remember how easy it is to forget the L-rd our G-d, and we ask Him, “Restore us to Yourself, L-rd, that we may return,” (Lamentations 5:21).