The month of Av is the fifth month on the Hebrew agricultural calendar (counting from Nisan). This month is considered the most tragic of all months for the Jewish people for many reasons. Many of these tragedies are very ancient in origin.
On the first day of this month Aaron the first high priest died (Numbers 33:38).
On the 9th day of the month it is believed that the Israelites, who originally left Egypt, were told by God that they would not be permitted to enter the promised land because of their unbelieve. They believed the report of the spies (not Joshua and Caleb) instead of the Word of God (Number 13-14).
On the 9th of Av the armies of Nebuchadnezzar are said to have destroyed the first Temple which was built by Solomon. Scripture, however, does not confirm the 9th of Av. It is very possible that it happened on that date as a burning of the city and Temple is recorded in scripture that happened in the 5th month somewhere on or between the 7th and 10th days of the month (see Jeremiah 52:12; Jeremiah 1:3; 2 Kings 25:8). This destruction of the city and Temple, I am sure, took more than one day. Jewish history records the 9th of Av as the date that the Temple was actually destroyed during this siege.
According to Jewish history the second Temple was also destroyed by the armies of Titus on the 9th of Av in 70 AD.
Because both Temples are said to have been destroyed on the 9th of Av, a three week mourning period known as "Three Weeks of Sorrow" are kept from the 17th day of the month of Tammuz and culminate on the 9th of Av (Tishah B'Av). A fast is observed on the 17th of Tammuz and on the 9th of Av. It is a time of remembrance and mourning for the people of Israel. It is a time that is supposed to call them to reflect and repent of sins that caused the destruction of Jerusalem, its Temple, and the exile of the people of Israel. During synagogue services on the 9th of Av, the book of Lamentations is read.
The prophet Zechariah speaks of the fast of the 4th, 5th, 7th and 10th months but declares that in the future their sorrow will be turned to joy and gladness as the coming King Messiah will bring peace and prosperity to the people once again; returning them to their homeland (Zechariah 8:19; 7:3-19).
Immediately after the 9th of Av the Jewish people have "seven weeks of comfort" in which they rejoice over the coming restoration of Israel; when she is once again living peacefully in her homeland and living out all of the promises made to Abraham.