The period in the Jewish calendar between the 17th day of Tammuz and the 9th of Av is called “The Three Weeks” or “Between the Straits – Beyn ha-m’tzarim.”
This is the period between two days of catastrophe in Jewish history. Among other incidents, the main tragedies were those related to the destruction of the Temples of old. The Roman siege of Jerusalem in 70 CE finally succeeded in breaking through the walls on the 17th of Tammuz. That day was declared a fast day. (The siege of Jerusalem during the First Temple breached the walls of the city on the 9th of Tammuz, and that was the original date for the fast, but it was moved to the 17th after the second destruction.) Intense fighting and terrible devastation followed for the nest three weeks until the Temple was set on fire and destroyed on the 9th of Av, the main fast day commemorating the loss of our Temple and the exile of our people from Israel. (The First Temple was also destroyed on the same day.)
The three-week period between these two days is a time of remembrance and of refraining from unnecessary celebration. Some people do not cut their hair as a sign of mourning.