Tisha B'Av (the Ninth day of Av),is the saddest day in the Jewish Calendar. This year 5772 (2012), we observe the fast day of Tisha B’Av starting from Shabbat, July 28 after nightfall, until nightfall on Monday, July 29.

"On the Ninth of Av, it was decreed on our fathers that they would not enter the Land (of Eretz Yisroel) [Bamidbar 14], the Temple was destroyed [both] the first time and the second time, Beitar (the stronghold of the Bar Kochba rebellion) was captured, and the city (of Yerushalayim) was plowed under."

Talmud Taanit 26b

On Tisha B’Av five tragedies happened to the Jewish people:

1. Hashem decreed that all the Jews who were saved from the Egyptian bondage will die out in the midbar (desert) and be deprived from entering Eretz Yisroel because of the sin of the meraglim (spies). (Bamidbar, Parshat Sh’lach, 14:26-38).
2. The first Bait Hamikdash, built by King Solomon, was destroyed on Tisha B’Av by the Babylonian King Nebuchadnezzer in 586 BCE.
3. The second Bait Hamikdash was destroyed on Tisha B’Av by Titus and his Roman soldiers in 70 CE.
4. in 135 CE, during the time of Rabbi Akiva, the Romans captured the fortress city of Beitar, the last stronghold of the leaders of the Bar Kochba revolt, and Bar Kochba along with tens of thousands of Jews were massacred.
5. Exactly a year later, the Roman Emperor Hadrian ordered the wicked Turnus Rufus to plow up the city of Yerushalayim, so that there should be no memory of the holy city. He then established a heathen temple on the site of the Bait Hamikdash and rebuilt Yerushalayim as a pagan city, renamed Aelia Capitolina, which the Jews were forbidden to enter.

Our Sages established this day, Tisha B’Av, as a fast day—a day of sorrow when we commemorate the destruction of Yerushalayim and the Bait Hamikdash. We must try to feel the gravity of the loss of the Bait Hamikdash and grieve the fact that we are still in exile today. We also remember all other tragedies that befell the Jewish people.