Tisha B'Av, also known as "The Ninth of Av," is a fast day that commemorates the destruction of the two Temples. It falls on the ninth day of the Hebrew month of Av, which usually coincides with late July or mid-August on the secular calendar.
The First Temple was built by King Solomon and was the most important place in ancient Judaism. It was destroyed when the Babylonians sacked Jerusalem in 586 B.C.E. The Second Temple was built on the site of the First Temple and was completed in 516 B.C.E. Sadly, the Second Temple was also destroyed, this time during the Roman siege of Jerusalem in 70 C.E. The destruction of the two Temples took place on the same day — the ninth of Av — about 656 years apart. These two events were so tragic that the ancient rabbis declared the anniversary of the Temples' destruction a day of mourning. This is the origin of Tisha B'Av.
The ninth of Av also happens to be the day that Jews were expelled from England in 1290, as well as the day that King Ferdinand and Queen Isabella expelled the Jews from Spain in 1492. Other tragedies have occurred on this day too. Any way you look at it, the ninth of Av has not been a good day for the Jewish people.
Tisha B'Av occurs in the Hebrew month of Av, but in a way it begins during the preceding month of Tammuz. On the 17th of Tammuz in 70 C.E. the Romans breached the walls of Jerusalem, then spent the next three weeks ransacking the city until the Second Temple was burned on the ninth of Av. In remembrance of this event Jews fast on the 17th of Tammuz and observe a time of mourning during the three weeks leading up to Tisha B'Av. No weddings are permitted during this period.
The Talmud says that "From the beginning of Av, we diminish happiness" (Mishnah Ta'anit 4:6). In keeping with this sentiment, the last nine days of "the three weeks" become increasingly mournful as observant Jews refrain from a number of prohibited activities. For instance, during these nine days Jews are not supposed to cut their hair or shave. This custom hearkens back to ancient times when a person showed they were in mourning by allowing their hair to grow.
During "the nine days" many Jews refrain from drinking wine, eating meat or participating in activities meant to be entertaining. Going to the movies, dancing or going out to a fancy dinner are examples of such pleasurable pastimes. According to the Talmud, Jews are not supposed to wash their clothes during this period either, because wearing clean clothes is an enjoyable experience. The purpose of all these prohibitions is to help people feel like true mourners by the time Tisha B'Av comes around on the ninth of Av.
Tisha B'Av is a full fast day, meaning that no food or drink can be consumed from one evening to the next. Pregnant women, nursing mothers and those who are severely ill are not allowed to participate in the fast because doing so would endanger their health. Observant Jews also refrain from bathing, wearing make-up or leather shoes (both symbols of luxury) or having sexual relations. Work is permitted on Tisha B'Av.
Synagogue services on Tisha B'Av are an emotional experience. During the evening service the book of Lamentations — a somber text about the destruction of the First Temple and the siege of Jerusalem — is read aloud, punctuated by sobs and wails from the congregation. Because people are in mourning, they don't greet each other at the synagogue and they sometimes sit on the floor instead of in seats. The following day, during the morning service, men continue to express their sorrow by refraining from wearing tefillin.