Av is the fifth month of the Jewish religious year, the eleventh of the civil
year. Av always has 30 days, and its zodiacal sign is Leo. The name Av is not
mentioned in the Bible, and is derived from the Akkadian language.
As Av is the month of the destruction of the Temples in Jerusalem, the rabbis
said: "When Av comes in, gladness must be dimished." The month is often referred
to as Menahem (Comforter) Av, either in reference to the divine
Father (av) who comforts His people following the destruction, or to
the Messiah who, tradition says, is to be born on the 9th of Av.
Between the straits
"Judah is gone into exile because of affliction, and because of great servitude;
she dwells among the nations, she finds no rest. All her persecutors overtook
her within the straits." 
The period of three weeks of mourning from the 17th of Tammuz
until the 9th of Av are known as the days "between the straits" (bein ha-mezarim),
based on the verse from Lamentations. The 17th day of Tammuz (known in Hebrew
as Shiv'ah Asar be-Tammuz) commemorates the breach of the walls of
Jerusalem by Nebuchadnezzar King of Babylon (586 BCE) three weeks before the
complete fall of the city and the destruction of the Temple.
During which a number of restrictions and abstinences (from public rejoicing,
buying or wearing new clothes, or eating meat) are traditionally observed.
On the Sabbaths during this period, the weekly portion from the Prophets
(the haftarah) warns of calamity, culminating in Shabbat Hazon
immediately prior to Tish'ah bi-Av, when the first chapter of the vision
(hazon) of Isaiah is read. On the Sabbath immediately after Tish'ah
bi-Av, Isaiah's great prophecy of consolation is read, and its opening
words "Comfort ye (nahamu), comfort ye, my people" give this
Sabbath its name of Shabbat Nahamu.
||  Lamentations
 "In the eleventh year of Zidkiya king of Judah,
in the tenth month, Nebuchadnezzar king of Babylon and all his army came
against Jerusalem, and they besieged it. In the eleventh year of Zidkiya,
in the fourth month, the ninth day of the month, a breach was made in
the city...." (Jeremiah 39:1-2)
||Ninth of Av:
a day of mourning
Fifteenth of Av: a day of rejoicing