The Three Weeks of Mourning begin Saturday evening, July 20th. It is a time of mournful reflection for the destroyed Bais HaMikdash and subsequent tragedies. In order to instill within ourselves a sense of loss, our Sages instituted numerous restrictions to create a feeling of sadness. The Talmud writes that one who mourns the destroyed Jerusalem will merit to see it rebuilt.
It is customary not to say the blessing of She’hechiyanu during the Three Weeks. Therefore, one should avoid eating fruits that they have not eaten for a year, as this would necessitate saying the blessing. In addition, one should not wear new clothing that requires making the blessing of She’hechiyanu. This includes new suits and new coats. One may purchase these items during the Three Weeks, it is wearing them for the first time that is a problem. One may purchase and wear any other type of clothing during the Three Weeks.
One may not cut hair during the Three Weeks. Waxing and eyebrow care is permitted.
One may not listen to lively music during the Three Weeks. This is true for live as well as recorded music. Most have the tradition to not listen to any music, even if it is not lively.
A Capella music is a matter of debate but there is what to rely upon to listen to such music.
Listening to music as a way of staying awake in a car, to help one concentrate or something of that nature is permitted.