Five things are forbidden on Tisha B'Av: Eating and drinking, washing, anointments, wearing of leather shoes, and marital relations.
Five tragedies occurred on this day: It was decreed that the generation of the desert would not enter Israel and would die in the wilderness. First and second Temples were destroyed. Beitar was captured. Jerusalem was plowed through after the Destruction.
The expulsion of Jews from Spain also took place on the Ninth of Av.
Nevertheless, this day is called, "Moed" and no Tachanun is said. This day will one day become a day of celebration.
We put on Tefillin at Mincha and recite the prayer נחם in שמונה עשרה. We begin to anticipate and prepare ourselves for the Redemption from after midday. We also begin the שבעה דנחמתא, the Seven Weeks of Comfort between Tisha B'Av and Rosh Hashana.