As the anniversary of the most tragic events in Jewish history, the Ninth of Av is the most important day of mourning in the Jewish year. Other than Yom Kippur, it is the only fast day in the year that lasts a full night and day. All other fasts begin in the morning and end that night. Tisha BeAv begins at sundown and continues till the following nightfall. Tisha BeAv carries several additional prohibitions that are not required by the other fasts.
It is customary to eat a meal before Mincha (afternoon prayers). This meal carries no restrictions. It is customary to eat well at this meal in preparation for the fast, but care must be taken not to overeat so that one can eat the Seudah HaMafseket comfortably. Tachanun is not recited during Mincha.
After the Mincha prayers it is customary to eat the last meal. This meal is called the Seudah HaMafseket (Separating Meal). It is forbidden to eat more than one cooked food at this meal. (Cooked includes any form of cooking even roasted, fried, or pickled.) Meat, wine and fish are forbidden. Intoxicating drinks should be completely avoided.
The meal is eaten sitting on the ground or a low seat. It is customary to eat a hardboiled egg (which serves as the cooked food). It is also customary to eat a piece of bread dipped into ashes and say, "This is the Tisha BeAv meal."
During the meal three men should not sit together so they will not have to recite the Birchat HaMazon (Grace after Meals) as a group. If they do eat together they still do not form a group.
When the eve of Tisha BeAv falls out on Shabbat, then none of these restrictions apply.
It is customary to remove the Paroches (curtain) from the Aron Hakodesh (Holy Ark) in the synagogue before Maariv (evening prayers). It is also customary to turn off the main lights in the synagogue and to pray by candlelight.
After Maariv is completed, the book of Eichah (Lamentations) is read aloud to the congregation. After Eichah is completed, the congregation recites Kinot, prayers of lamentation.
It is proper for a person to sleep in a less comfortable manner than he is accustomed to. If he usually sleeps with two pillows then he should sleep with only one. Some have to custom to sleep on the ground on the night of Tisha BeAv and to rest their head on a stone.
After the Torah reading the congregation recites Kinot. This should last till a little before noon. After Kinot the prayers are completed. Lamenatzeach and the second verse of Uvo LeTzion are skipped. Some do not say Shir Shel Yom now but wait till Mincha.
It is proper for every person to read the book of Eichah again. After the hour of noon it is permissible to sit on an ordinary seat. At Mincha we don talit and tefillin. The Torah is taken out and the standard portion and haftarah for fast days is read.
During Shemoneh Esrei the following prayer is inserted in the blessing of VeLeYerushalayim Ircha:
G-d our God, console the mourners of Zion and the mourners of Jerusalem, and the city that is mournful, destroyed, shamed, and desolate. Mournful without her children, destroyed without her residences, shamed without her honor, and desolate without inhabitant. She sits with her head covered, like a barren woman who does not give birth. She has been devoured by the legions, and conquered by the worshipers of foreign powers, and they put your people, Israel, to the sword and willfully murdered the devout [servants] of the High One. Therefore Zion cries bitterly, and Jerusalem raises her voice, "My heart! My heart [aches] on the slain! My stomach! My stomach [aches] on the slain!" For You, G-d, with fire you burned her, and with fire you will rebuild her, as it is said, "And I will be for her, says G-d, a wall of fire around her, and I will be a glory within her."(Zechariah 2:9) Blessed are You, G-d, Who consoles Zion and builds Jerusalem.