The Three Weeks are a fast during the month of July which was the original basis for the Apostles' Fast.
The Three Weeks are a traditional period of Fasting in Judaism to mourn various disasters which have taken place throughout Jewish history starting with the Sin of the Golden Calf.
The last day of the fast is Tisha bAv which is the day we remember the destruction of the 1st and 2nd Temples mourning the success of the wicked Kingdom in 70CE by fasting to purify ourselves as the Temple of the New Jerusalem.