First printing of the Zohar, the fundamental work of the Kabbalah (Jewish esoteric and mystical teachings), authored by the Talmudic sage, Rabbi Shimon Bar Yochai.
Links: The Kabbalah
Av 20 is the anniversary of the passing of the Lubavitcher Rebbe's father, Rabbi Levi Yitzchak Schneerson (1878-1944), in Alma Ata (currently Almaty), Kazakhstan. Rabbi Levi Yitzchak was Chief Rabbi of Yekaterinoslav (currently Dnipro), Ukraine, and was arrested and exiled to Kazakhstan by the Stalinist regime as a result of his work to preserve Jewish life in the Soviet Union.
Links:
A Brief Biography
Rebbetzin Chana Scneerson's biography, which includes her account of her husband's heroism, arrest and passing.
In our times, it is crucial that every Jew who knows anything must be a teacher to others.
If you know alef-bet, teach alef-bet.
If you know only alef, find someone to teach alef.
But all must teach.