To put it simply, Tisha B’Av is the saddest day of the Jewish year. It occurs on the ninth day of the Hebrew month of Av, which is usually in July or August. Tisha B’Av commemorates the destruction of the the First and Second Temples in Jerusalem in 586 B.C.E. and 70 C.E., and is observed as a day of communal mourning.
To observe Tisha B’Av, Jews traditionally fast and avoid fun activities (from listening to music to washing to wearing leather shoes). There are many sad psalms and poems that are read, including the biblical Book of Lamentations.
Since it is a day of such sadness, a three week period of low-level mourning leads up to the holiday of Tisha B'Av. During this period it is traditional to refrain from public celebrations, such as weddings. The nine days just before Tisha B’Av are an even more intense period of mourning, where traditional Jews refrain from eating meat.