On the Ninth of Av of the year 2449 from creation (1312 BCE), the generation of Jews who came out of Egypt under Moses' leadership 16 months earlier were condemned to die in the desert and the entry into the Land of Israel was delayed for 40 years.
As related in Numbers 14, when the Spies that Moses sent to the Land of Canaan returned with their disheartening report (see "Today in Jewish History" for yesterday, Av 8), the people wept all night -- the night of Av 9th -- proclaiming that they'd rather return to Egypt than attempt to conquer and settle it; G-d decreed that the entire generation would wander in the desert for 40 years until the last of them died out, and that their children, under the leadership of Joshua, will enter the land He promised as Israel's heritage.
This is the first of five national tragedies that occurred on Av 9 listed by the Talmud (Taanit 4:6), due to which the day was designated as a fast day. The other four are: the destruction of the two Temples, the fall of Betar, and the plowing over of Jerusalem. (see below)
Links: The Spies
Both the first and second Holy Temples which stood in Jerusalem were destroyed on Av 9: the First Temple by the Babylonians in the year 3338 from creation (423 BCE), and the second by the Romans in 3829 (69 CE).
The Temples' destruction represents the greatest tragedy in Jewish history, for it marks our descent into Galut--the state of physical exile and spiritual displacement in which we still find ourselves today. Thus the Destruction is mourned as a tragedy that affects our lives today, 2,000 years later, no less than the very generation that experienced it first hand.
Yet the Ninth of Av is also a day of hope. The Talmud relates that Moshiach ("anointed one"--the Messiah), was born at the very moment that the Temple was set aflame and the Galut began. [This is in keeping with the teachings of our sages that, "In every generation is born a descendent of Judah who is worthy to become Israel's Moshiach" (Bartinoro on Ruth); "When the time will come, G-d will reveal Himself to him and send him, and then the spirit of Moshiach, which is hidden and secreted on high, will be manifested in him" (Chattam Sofer).]
Links:
The Holy Temple: an Anthology
Moshiach: an Anthology
Moshiach and the Future Redemption
See "Laws and Customs"
Betar, the last stronghold in the heroic Bar Kochba rebellion, fell to the Romans on the 9th of Av of the year 3893 (133 CE) after a three-year siege. 580,000 Jews died by starvation or the sword, including Bar Kochba, the leader of the rebellion.
Link: A Talmudic account of the fall of Betar
On this date in 1290, King Edward I of England issued an Edict of Expulsion, ordering the expulsion of all Jews from his territory.
The Jews of Spain were expelled by King Ferdinand and Queen Isabella on the 9th of Av of 1492, terminating many centuries of flourishing Jewish life in that country.
Passing of R. Yaakov Yitzchak Horowitz, the “Seer” of Lublin (1815)
R. Yaakov Yitzchak Horowitz (1745–1815), known as the “Seer of Lublin,” was the successor to R. Elimelech of Lizhensk (1717–1787), and a major personality in the spread of the chassidic movement throughout Poland. Many of the great Chassidic masters of the time were his disciples. Many of his insights were published posthumously in Divrei Emmet, Zichron Zot, and Zot Zichron.
Link: The Clock, Crossroads Puzzle, The One That Nearly Got Away
Mourning the destruction of the Temple and the exile of Israel, (see "Today in Jewish History") we abstain from eating and drinking, bathing, the wearing of leather footwear, and marital relations--for the night and day of Av 9 (i.e., from sundown on Av 8 to nightfall on Av 9). It is customary to sit on the floor or a low seat until after mid-day. Torah study is restricted to laws of mourning, passages describing the destruction of the Temple, and the like. The tefillin are worn only during the afternoon Minchah prayers. (For more laws and customs see link below.)
Tachnun (confession of sins) and similar prayers are omitted.
What does fasting on Yom Kippur achieve?
That even our physical bodies can feel the precious closeness of this day.
Yom Kippur is such a holy day, the body receives life from fasting.