In Judaism, Tish'a B'Av, which is an annual day in observance of a number of disasters in Jewish history, is traditionally observed with a fast.
On this day, approximately 2500 years ago, the Babylonians destroyed the first Holy Temple in Jerusalem, and several centuries later on the same day the Romans destroyed the second Holy Temple in Jerusalem.
Because of its association with disasters in Jewish history, this is also used to remember other tragedies which have befallen the Jewish people, such as the Holocaust.
Here are some ways you can observe this day according to Jewish tradition: