Every evening, the priest would kindle the seven lights of the menorah in the Holy Temple. Miraculously, although six of the seven candles would burn out, the western lamp would remain lit until the following evening. (See link below for the exact identity of the western lamp.)
During the reign of the idolatrous King Ahaz (father of the pious King Hezekiah), this miracle discontinued. The first time the western lamp was found to have extinguished was on 18 Menachem Av (or, according to other versions, 17 Menachem Av). (Shulchan Aruch, Orach Chaim 580:2)
Link: Spiritual Space
Sixty-seven Jewish men, women and children were slaughtered, and scores wounded, raped and maimed, by their Arab neighbors in the city of Hebron, who rioted for three days amid cries of "Slaughter the Jews." The killings began on Friday afternoon, 17 Av, and most of the victims lost their lives on Shabbat, 18 Av. The survivors were forced to evacuate to Jerusalem, and the ancient Jewish community of Hebron, which had lived in relative peace in the city for hundreds of years, was not revived until after Israel's capture of Hebron in the 1967 Six Day war.
Link: The Hebron Massacre
During the summer months, from the Shabbat after Passover until the Shabbat before Rosh Hashahah, we study a weekly chapter of the Talmud's Ethics of the Fathers ("Avot") each Shabbat afternoon; this week we study Chapter Four.