The three weeks between the 17th of Tammuz and Tisha B’Av (commonly known as “The Three Weeks”) are a time of mourning for the Bnei Yisrael. We therefore refrain from various joyous practices during this period. They are split into three levels with their intensity increasing as we approach the ninth of Av. This article deals with the first of these levels from the 17th of Tammuz up until Rosh Chodesh Av.
We don’t take haircuts or shave. This is similar to Sefirah and more details can be found in this post.
We don’t listen to music. One who is professional musician as well as one who is learning how to play an instrument may continue to do so. (After Rosh Chodesh Av it is best to do so in a secluded place). For more music related laws please see the Sefirah post.
We don’t get married during this time.
We do not recite Shecheyanu. Therefore one shouldn’t purchase an item that requires a Shehechyanu, such as new clothing, a new car (for personal use, not business or family use), or a new Tallis. New clothing that does not require Shecheyanu or that was bought before the 17th of Tammuz and Shecheyanu was already recited may be worn for the first time up until Rosh Chodesh Av.
Clothing that requires Shecheyanu may not be bought but may be worn on Shabbos until Rosh Chodesh.
Rav Soloveitchik had a fascinating Halachic observation: All laws of Aveilus throughout the year must be modeled after the Aveilus one has for a relative that passes away. Therefore we will treat Tisha B’Av as Shiva, the Nine Days as Shloshim, and the three weeks are analogous to the 12 months following the death of a parent. (Nefesh Harav).