In Judaism we have a tradition called yahrzeit, that we commemorate the anniversary of a relative's death on the Hebrew calendar. Had we commemorated the all the bad things that have happened to the Jewish people on their proper yahrzeit, the entire year would be spent in mourning. Therefore, One day in 354 (or 353, 355, 383, 384, or 385 depending on the year) we aggregate all of the horrible things that have befallen the Jewish people (and there have been quite a few of them...) into a single 25 hour period. So many catastrophes beyond measure occured on this day itself: the sin of the 10 spies and the 40 years of the dying out of the slave generation, the destruction of the first and second temples, the destruction of Beitar and the proscription on the burial of the martyrs, the expulsion of the Jews from Spain and the Spanish Inquisition, the dawn of World War I (events which led to the Holocaust), the expulsion from Gush Katif, and more.
On Rosh Hashannah we declare "Hayom Harat Olam", "Today the world was created". On Tisha B'Av we should say "Today the world was destroyed". Today was the day when the world lost its innocence, where the world lost the central place of worship, where the world lost its access to the Divine Presence of the Shechinah and the ability to bask in holiness and purity. Our happiest of occasions are diminished: On Pesach we are unable to observe the main mitzvah: to eat the Pesach lamb. We don't get the Avodah on Yom Kippur, we mourn Jerusalem even at the moment of highest joy at weddings.
But we may not mourn excessively! During the Nine Days we are forbidden to eat meat or imbibe wine. Soon after the Destruction, a group of people vowed not to eat meat or drink wine everyday until the Third Temple was built. The rabbis asked them why they vowed this and they responded that meat was offered and wine was libated in the Temple so why should they enjoy these things without a Temple in which to offer them. The rabbis then informed them that they shouldn't eat bread or drink water as these were waved and libated in the Temple. The people saw the point of the rabbis, that mourning excessively would cause them to starve to death, and they decided not to abstain from meat or wine year round.
But today we concentrate our mourning. We mourn for "the compassionate women who boiled their children and ate them at the destruction of the daughter of my people" (Lamentations 4:10). for this and all of the horrible things our people have been reduced to throughout history. "Pour out your heart like water before the Face of God" (2:19). Though the gates of heaven are closed shut, they soon burst open and the promise of redmption is strong. God who took us from slavery to freedom, from hopelessness to redemption, from grief to joy, from mourning to festivity. The last line was: meevel lYom Tov. Our Rabbis say that this day of mourning will be transformed into a Yom Tov when the Temple is rebuilt and that it may be that the Messiah will be born on this very day of Tisha B'Av. Scattered throughout the depravity of Eicha, there is scattered some hope (3:21, 3:56-66, 5:19-21). Today we mourn. Tomorrow may we be redeemed.