Tisha B'Av is at the end of a three-week period of mourning which began  with the Fast of 17 Tammuz, the date on which the outer walls of the city of Jerusalem were breached during the siege. 

Several bad events happened to the Jewish people through our history on this date: Moses broke the first tablets of the Law when he came down from Mt. Sinai after 40 days - to find the people worshipping the Golden Calf. It is the date on which the Betar stronghold fell during the Bar Kochba revolt, the date of the Jewish expulsion from Spain in 1492, the beginning of Nazi deportations of Jews from the Warsaw Ghetto.

The day is marked publicly in the State of Israel by the closure of restaurants. 

The Book of Lamentations is traditionally read, the Kinot, a 25 hour fast, deprivation of comfort and physical contact. 

In Jerusalem, thousands of people stream towards the Kotel, the Western and only remaining Wall of the Second Temple to commemorate the destruction and pray for redemption.

Restrictions for the fast are like those of Yom Kippur: Not eating or drinking (even water), not washing or shaving, not wearing leather shoes, and studying Torah. Many of the customs are like those that we use for mourning a close relative. We don't smile or laugh and we sit on low stools.

All of the restrictions of Tisha B'Av begin at sundown. It is customary to remove the curtain from the Holy Ark in the synagogue before evening prayers (some congregations put up black curtains as a sign of mourning). It is also a tradition to pray by candlelight during Maariv (the Evening Prayers) on this night. After evening prayers the Book of Lamintations is read and prayers of Lamentation are recited.

On the day of Tisha B'Av, Tallit and Tefillin are not worn during morning prayers (the are worn during afternoon prayers). The Torah Portion that is read is Deuteronomy 4:25 - 40 and the Haftara is Isaiah 8:13 - 9:23. During the afternoon prayers Tallit and Tefillin are worn, the Torah is taken out and the standard portions are read.

Tisha B'Av ends at nightfall, but even though the fast has ended we do not eat meat or celebrate the ending of the fast day. This is because the Temple continued to burn into the tenth day.

When the Ninth of Av falls on Shabbat, the fast is postponed until Sunday.