6 Menachem Av 5772
Counting from Tishrei, Av is the eleventh month. The ninth day of this eleventh month has seen horror upon horror befall the Jewish people. So, you could say it's the 9/11 of the Jewish calendar - only multiplied by millions.
Here is a history of events which took place on Tisha b'Av courtesy of Ohr Somayach:
2448 (1312 bce)Spies return from 40 days in Israel with evil reports of the Land of Israel. Jewish people cry in despair, give up hope of entering the Land of Israel.
3340 (421 bce)Destruction of First Temple by the Babylonians, under Nebuchadnezar. About 100,000 Jews killed during invasion. Exile of remaining tribes in southern kingdom to Babylon and Persia.
3830 (70 ce)
Destruction of Second Temple by Romans, under Titus. Over 2,500,000 Jews die as a result of war, famine and disease. Over 1,000,000 Jews exiled to all parts of the Roman Empire. Over 100,000 Jews sold as slaves by Romans. Jews killed and tortured in gladiatorial "games" and pagan celebrations.
3892 (132 ce)Bar Kochba revolt crushed. Betar destroyed - over 100,00 killed.
3893 (133 ce)Turnus Rufus ploughs site of Temple. Romans build pagan city of Aelia Capitolina on site of Jerusalem. [The myth of Palestine began here.]
4855 (1095 ce)
First Crusade declared by Pope Urban II. 10,000 Jews killed in first month of Crusade. Crusades bring death and destruction to thousands of Jews, totally obliterate many communities in Rhineland and France.
5050 (1290 ce)
Expulsion of Jews from England, accompanied by pogroms and confiscation of books and property.
5252 (1492 ce)Inquisition in Spain and Portugal culminates in the expulsion of the Jews from the Iberian Peninsula. Families separated, many die by drowning, massive loss of property.
5674 (1914 ce)Britain and Russia declare war on Germany. First World War begins. First World War issues unresolved, ultimately causing Second World War and Holocaust. 75% of all Jews in war zones. Jews in armies of all sides - 120,000 Jewish casualties in armies. Over 400 pogroms immediately following war in Hungary, Ukraine, Poland and Russia.
5702 (1942 ce)Deportations from Warsaw Ghetto to the Treblinka concentration camp begin.
5749 (1989 ce)Iraq walks out of talks with Kuwait.
5754 (1994 ce)
The deadly bombing the building of the AMIA (the Jewish community center in Buenos Aires, Argentina) which killed 86 people and wounded some 300 others.
[I would add]
5765 (2005 ce)
The forcible expulsion of the approx. 8500 Jews of Gush Katif and the destruction of all Jewish property there (including synagogues and graves) began.
And which was the defining event which gave this date its energy to bring more horror in its wake?