Tisha B'av is a day of mourning, a day that is remembered for a number of gloomy events in Jewish history. On that very day, the First Temple was demolished in 586 BCE and so was the second temple in 70 CE. Again, this date also holds the start of the exclusion of the Jews from Spain. Tisha B�Av is most significant amongst the other three days of mourning in relation to the destruction of the First and Second Temples.
It is found that, it all starts with the Bein ha-Metsarim - a mourning period on the 17th of Tammuz, about three weeks before Tisha B�Av. This was the day when the walls of Jerusalem were demolished before the Second Temple was put to pieces. Mourning practices as well as fasting are seen here on this day. No weddings and religious Jews indulge in cutting their hair or listening to music.
Like the Day of Atonement, this holy day stretches from sunset on the eve of the 9th of Av till sunset the next day. There are restrictions on physical pleasures on this day. There are other prohibitions as well like washing, wearing leather shoes, etc. Moreover, sleeping on the ground and sitting on a chair at daytime are to be done away with.
Reading the Book of Lamentations is required whatsoever. The book tells about the fed-up state of Jerusalem and the Jewish people after destruction. It also gives explanation for the diktat of destruction issued by God against the Jewish people�s sins. The Lamentations are to be read being seated on the floor and by the candlelight.
The Jews mourn as they recall all their ancestors who were thrown out from the land of Israel. In fact, after the First Temple was destroyed, the Jews were exiled to Babylonia only allowing them to return to their homes some fifty years later. What followed then was the expulsion of the Jews, which was just after the Second Temple, was demolished.
It is observed that, before the holiday actually commences, having light dinner falls within the custom. And dinner takes in hard-boiled eggs and lentils. They are to be had on the days of mourning.
Thus, you will find that Tisha B�Av is the day where the Jews demonstrate their grief dedicated to the myriad tragedies in Jewish history.