The Three Weeks
The destruction of the Temple in Jerusalem is commemorated with a period of mourning.
The three-week period in summer that begins with the fast of the 17th of Tammuz and climaxes with Tisha b'Av is known simply as "The Three Weeks." It is a time of grieving for the destruction of both the First and Second Temples in Jerusalem. This mourning period was first mentioned in the biblical Book of Zechariah in the Prophets--and, since then, it has been observed as a period of sadness.
The Multiple Tragedies
The 17th day of the Hebrew month of Tammuz is a date in which many tragedies and pitfalls happened, according to the Mishnah (Taanit 4:6). It is traditionally believed to be the date that Moses broke the original Ten Commandments after coming upon the Israelites as they worshiped the Golden Calf. The Roman rulers forbade sacrifices to be made in the Second Temple on this date in 69 C.E., and, in the following year, the walls of the Old City of Jerusalem were breached. This attack led to the destruction of the Temple three weeks later.
In Hebrew, the period of the Three Weeks is known as "bein hametzarim," or, literally, "within the straits" or "within the borders." This name comes from a verse in the Book of Lamentations, or Eicha, which is read on Tisha B'Av: "Judah has gone into exile because of affliction, and because of great servitude. She dwelt among the nations, she found no rest; all her pursuers overtook her within the borders." (1:3) This idea of borders--or "restrictions"--alludes to the additional restrictions of mourning which are traditionally taken on during this period.
Traditionally, Jews take on several mourning customs during the Three Weeks. Similar to the period of the Omer, no weddings, parties, or public celebrations are held. Some people abstain from getting haircuts and shaving. Some people also refrain from going to concerts or listening to music during this period.