Today is the 17th day of the Hebrew month of Tammuz - a day when many Jews remember the siege of Jerusalem in 70 C.E. by fasting.
On this day in Jewish history the Roman Empire succeeded in breaching the walls of Jerusalem in response to the Jewish Great Revolt. The Great Revolt took place from 66 to 70 C.E. and was the first of three major Jewish rebellions against the Romans. Once the Romans entered the city they spent three weeks ransacking it, which eventually resulted in the destruction of the Second Temple on the 9th day of the Hebrew month of Av. This tragic loss is also remembered by a fast day known as Tisha B'Av (the 9th of Av).
You can learn more about the Great Revolt in: What Was the Great Revolt?