The 17th of Tammuz is the first day of “The Three Weeks”, the annual period of Jewish mourning. The 17th of Tammuz is a fast day, (pre-dawn until after dark, as opposed to Tisha B’Av which is a little more than 24 hours). As is always the case, drinking is prohibited as well.
In the year 69, on the 17th of Tammuz, the Roman legions were able to breach the walls of Jerusalem, sealing its fate. It would take three weeks for the Romans to reach and burn the Temple, which they did on the 9th of Av.