Five incidents befell our ancestors on the Seventeenth of Tammuz, and five on the Ninth of Av. On the Seventeenth of Tammuz, the Tablets were broken, the con-tinual [daily] sacrifice was discontinued, the city [wall] was breached, Apostumos burned the Torah, and placed an image in the Sanctuary.  On the Ninth of Av, it was decreed upon our ancestors that they would not enter the Land, the Temple was destroyed for the first time and the second time, Bethar was conquered, and the city was plowed under.  Therefore, when Av begins, we curtail joy.