Tisha B’Av marks the destructions of the Temples in Jerusalem, the first by the Babylonians in 586 BCE and second by the Romans in 70 of the Common Era. Each is said to have occurred on the same day, the ninth of Av on the Hebrew calendar. The day is commemorated as a fast day and has subsequently been connected to the fall of Beitar ending the Bar Kochba rebellion in 135 AD and the expulsion of the Jews from England (1290) and from Spain (1492).
Since the ninth day of Av is the date on the Hebrew calendar, its observance moves around on the Gregorian calendar.
When Tisha B’Av falls on Shabbat its observance is moved to the next day, Sunday, because Shabbat is the most important Jewish holiday and no mourning is permitted to violate the joy of Shabbat. As with all Jewish holidays, the observance begins in the evening so will begin Saturday after sundown.