The Three Weeks is an annual mourning period that falls out in the summer. This is when we mourn the destruction of the Holy Temple and the beginning of our still-ongoing Exile.
This year, the three weeks began on Tuesday, July 23 (Fast of 17th of Tammuz) and will conclude on Tuesday, Aug 13 (Fast of Tisha B'Av).
The period begins on the 17th of the Hebrew month of Tammuz, a fast day that marks the day when the walls of Jerusalem were breached by the Romans in 69 CE.
It reaches its climax and concludes with the fast of the 9th of Av, the date when both Holy Temples were set aflame. This is the saddest day of the Jewish calendar, and it is also the date of other tragedies spanning our nation's history.