The Nine Days of Av are a religious observance in Judaism that takes place during the first nine days of the Jewish month of Av (corresponding to July/August). The Nine Days begin on Rosh Chodesh Av ("First of Av") and culminates on the public fast day of Tisha B'Av ("Ninth of Av").
The Nine Days are part of a larger period of time known as The Three Weeks, which begin with the public fast day of the Seventeenth of Tammuz — commemorated in Judaism for the time when the forces of Nebuchadnezzar of Babylonia broke through the defensive walls surrounding Jerusalem, generally accepted as happening in 586 BCE — and end with the public fast day of Tisha B'Av — when the Babylonians destroyed the First Temple in 597 BCE and when the Second Temple was destroyed by the Romans in 70 CE. During the entire Three Weeks, certain activities are forbidden to Jews by Jewish law in order to decrease joy and inspire mourning over the destruction of Temple.
The Talmud says, "When the month of Av begins, we [i.e. Jews] reduce our joy." The Nine Days inaugurates an even greater level of communal and personal mourning in recognition of the many tragedies and calamities that befell the Jewish people at this time. These tragedies include the destruction of both Jewish Temples in Jerusalem, the expulsion of the Jews from Spain on Tisha B'Av 1492, and the outbreak of World War I on Tisha B'Av 1914, which destroyed many Jewish communities. The Nine Days are considered an inauspicious time, fraught with danger even in our day and age.