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
Take these words of Mine to your hearts and to your souls…teach them to your children so that this will be what they speak about, when you are sitting at home, walking on your way, lying down to rest, and arising from your sleep. (Deuteronomy 11:18-19)
The simple meaning of this instruction is as follows:
You must teach your children Torah.
Not as a burden, but as a love; so that they will cherish Torah wisdom, share it, and speak about it at every time of day, in every circumstance. Until it becomes the defining activity of their lives.
And to accomplish this, you too must be engaged, heart and soul, in your children's education the entire day.
Not only at home, when you are in your own space, but even when you're off at work or in other circumstances you do not control, this is what should preoccupy your mind.
So that when you lie down to rest after a long, hard day, the concern won't go away.
And when you awaken refreshed in the morning, your first thought will be, "What's happening with my children's education?"
This is why there are Jews today, how this miracle occurred: Because there were Jewish parents who focused their entire lives on the Torah education of their children.