The month of Av begins with sorrow as we approach Tisha B’Av (9th Av), which is the anniversary of the destruction of both Temples and our exile from the land of Israel. Fittingly, it is also the anniversary of the twelve spies returning to Moses in the wilderness with the report that the Promised Land would be impossible for the Israelites to take; because of this, God condemned the Children of Israel to 40 years more wandering.
The days leading up to the fast see the measure of mourning intensified. From Rosh Chodesh, one should not eat meat or drink wine, except on Shabbat. One does not launder clothes, swim or bathe for pleasure. It is considered an inauspicious time for court cases and business ventures. However, there is no mourning on Shabbat; one can wear clean clothes, eat meat and drink wine - even when Tisha B’Av falls on Shabbat. When this happens, the fast is postponed to Saturday night/Sunday, beginning at the appropriate time, even before Shabbat has gone out.
The Fast is preceded by the Seudat Hamafseket, a simple meal, normally a roll and a boiled egg dipped in ash. The fast is from dusk the night before through the entirety of the day.
The Book of Lamentations (Eicha) is read in the evening together with Kinnot or liturgical dirges. The same prohibitions as Yom Kippur apply (the Shabbat/Yom Tov prohibitions on cooking, electricity, riding etc do not). Even the pleasure of Torah study is denied.
Tallit and Tefilin are worn at Mincha in place of Shacharit.
The fast ends at nightfall but as the Temple burned through the following day, the prohibitions on wine, meat and haircuts remain in force till 10th Av.
Pictured is a depiction of Roman soldiers carrying the spoils from the Temple in Jerusalem after its destruction in 70 CE. Image is from the Arch of Titus in Rome.