The next upcoming Tisha B'Av starts on sundown of Monday August 12 ending at sundown on Tuesday, August 13.
Celebration\ Observance
Many will fast, pray and abstain from pleasure seeking activities as well as not wash or bath.
History
The Babylonians in 586 BCE destroyed the First temple and later the Romans in 70 CE destroyed the second temple all on the ninth day of Av creating the remembrance day of Tisha B'Av. There are other years were other tragedies occurred on the same day and are also part of the overall commemoration of the day.