Tisha B'Av

Introduction
Tisha B'Av (Lit. 9th of Av) falls on the 9th day of the Jewish month of Av. Tisha B'Av is a fast day commemorating the destruction of the first and second Temples in Jerusalem.

On Tisha B'Av one may not eat, drink, have marital relations, wear leather shoes or bathe.

What happened on Tisha B'Av
Tisha B'Av commemorates 5 tragedies that befell the Jewish people on that date:
 * 1) It was decreed that the generation which left Egypt would remain in the desert for 40 years and not enter the land of Israel, after believing the inaccurate report of 10 of the 12 spies in the year 2449.
 * 2) The first Bet Hamikdash (Holy Temple) was destroyed on 9 B'Av in the year 3339.
 * 3) The second Bet Hamikdash (Holy Temple) was destroyed on 9 B'Av about 1948 years ago.
 * 4) The city of Betar was captured and tens of thousands of Jews were killed in the year 3893.
 * 5) The wicked Turnus Rufus plowed the site of the Bet Hamikdash and its surroundings and renamed it Aelia Capitolina, also in the year 3893.

Since these tragedies occurred on 9 B'Av, it was decreed as a day of fasting and mourning.

Other tragedies that happened on 9 B'Av:
 * 4,000 Jews were expelled from England by King Edward I in the year 5050 (18 July 1290)
 * 300,000 Jews were expelled from Spain by Isabella of Castile and Ferdinand II of Aragon in the year 5252 (2 August 1492)
 * World War 1 started in 5674 - 1 August 1914 - with Germany declaring war on Russia
 * 8.000 Jews forcibly removed from their homes in Gush Katif by the Israeli government (15 August 2005)

Fasting
On Tisha B'Av almost everybody needs to fast.

Pregnant and nursing mothers need to fast on 9 B'Av even if it causes them discomfort, unless it's dangerous to their health.

During the first 7 days after childbirth, a mother is not allowed to fast.

Anybody who isn't healthy should only fast for a few hours. This includes a mother between 7 and 30 days after childbirth unless she feels up to fasting.

Children are not allowed to fast.

Those who are not fasting should limit their food intake to the bare minimum; only bread and water if possible.

On the fast of 9 B'Av it is customary not to sit on chairs from the time the fast begins (next week, Wednesday afternoon before sunset) until noon the following day (Thursday).

Sitting low
Instead, one sits on the floor.

Anything within 3 Tefachim (about 9" - 24 cm) of the floor is considered as being on the floor.

After noon one may sit on regular chairs. However, all other restrictions of the fast apply until the fast is over at night-fall.

Work on Tisha B'Av
One may do "minor" work on 9 B'Av, such as turning on lights and driving.

Any work that takes times, as well as all business dealings, should not be done until noon, so as not to get distracted from mourning the destruction of the Bet Hamikdash - the Holy Temple in Jerusalem.

While one may go to work and open ones business on 9 B'Av afternoon, it's commendable not to.

One may have a non-Jew do ones work on 9 B'Av, and one may do any work needed to prevent a monetary loss.

Leather shoes
One does not wear leather shoes on Tisha B'Av, and therefor one must remove them before sunset.

One may wear shoes that have no leather in them; cloth, rubber and wood are OK.

Other clothing items made of leather are permitted.

Source
The laws of Tisha B'Av are documented in:
 * Shulchan Aruch, Orach Chaim 553 to 559
 * Kitzur Shulchan Aruch 122 - 124