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 yahtzeit (anniversary of the passing) of the Lubavitcher Rebbe's father, Rabbi Levi Yitzchak Schneerson (1878-1944), in Alma Ata, Kazakhstan. Rabbi Levi Yitzchak was Chief Rabbi of Yekaterinoslav (currently Dnepropetrovsk), 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.