The 17th of Tammuz, which falls on Thursday, is the start of the days known as the "three weeks", which end on Tisha B'av. These days are considered days of mourning because it is when in the year 69 CE the walls of Jerusalem were and the second Temple was destroyed.
More stories:
It is true that our generation was privileged to see the redemption of land of Israel: the establishment of the state, Israel's success in wars with its neighbors and the liberation of Jerusalem and the Temple Mount. And yet it is clear to all of us that this is not enough, and there is much more to be done for complete redemption.
Therefore, this year too we will mourn the destruction of the Temple, and for everything that is still missing, out of hope for the rebuilding of the Temple and Jerusalem in their entirety, soon.
On the 17th of Tammuz, the forces of the Roman general Titus, breached the walls of Jerusalem during the Second Temple period. The battles within the city lasted for three weeks, until the ninth of Av, when the Temple was destroyed.
The 17th of Tammuz symbolizes the beginning of the active destruction that gave rise to the 2,000-year exile, and all the disasters that happened during them to the people of Israel.
Sages established appropriate behavior for these days:
fasting on the 17th of Tammuz and Tisha B'av, and the behavior of mourning on the other days in between – such as not listening to music and, for the last nine days, refraining from eating meat and drinking wine, except on Shabbat.
There is a difference in the severity of the days of the fast: since on Tisha B'Av the temple was burned down, and it expresses the peak of the destruction, the fast on it is more severe and starts in the evening. On the 17th of Tammuz the fast began at dawn on Thursday, and ends when the stars come out.