When reckoning from Tishrei, the month of Av is the eleventh month of the year, and when reckoning from Nisan, as is customary in the Tanakh, it is the fifth month.
This month includes the saddest day of the Jewish calendar—the 9th of Av: a day of fasting and mourning commemorating the destruction of the first and second Temples in Jerusalem. Therefore, the month of Av is often referred to with the prefix "menachem" (Menachem Av), meaning "Comforter."
This month also includes a holiday: the 15th of Av.