The ‘Three Weeks’ is an annual communal mourning period between the fasts of Tammuz and Av inclusive. During this time, we mourn the destruction of the Holy Temples in Jerusalem, along with associated tragedies, as well as reflecting on our continuing exile from the Land of Israel. The most intense part of this period is the ‘Nine Days’, from Rosh Chodesh Av (the start of the month of Av) until Tisha B’Av itself inclusive.
The Talmudic and subsequent rabbis instituted a number of ‘mourning’ practices during the three weeks to help us focus on the lessons and ramifications of this time of year on Jewish history.
In Ashkenazi communities, the practice is to abstain from haircuts, shaving, listening to performance music, saying the ‘shehecheyanu’ blessing, celebrating joyous occasions such as weddings or buying expensive new items (unless for a mitzvah or if the clothes will not be available subsequently or if you will miss a major sale) for the whole of the three weeks.