Rabbi Isaac Luria Ashkenazi, known as Ari HaKadosh ("The Holy Lion") passed away on the 5th of Av of the
year 5332 from creation (1572 CE). Born in Jerusalem in 1534, he spent many years in secluded study near Cairo, Egypt. In 1570 he settled in Safed, where he lived for two years until his passing at age 38. During that brief period, the Ari revolutionized the study of Kabbalah, and came to be universally regarded as one of the most important figures in Jewish mysticism. It was he who proclaimed, "In these times, we are allowed and duty-bound to
reveal this wisdom," opening the door to the integration of the teachings of Kabbalah--until then the province of a select few
in each generation--into "mainstream" Judaism.
During the “Nine Days" from Av 1st to the Ninth of Av, we mourn the destruction of the Holy Temple. We abstain from meat and wine, music, haircutting, bathing for pleasure, and other joyous (and dangerous) activities. (The particular mourning customs vary from community to community, so consult a competent halachic authority for details.)
Consumption of meat and wine is permitted on Shabbat, or at a seudat mitzvah (obligatory festive meal celebrating the fulfillment of certain mitzvot) such as a brit (circumcision), or a siyum celebrating the completion of a course of Torah study (i.e., a complete Talmudic tractate). The Lubavitcher Rebbe, of righteous memory initiated the custom of conducting or participating in a siyum on each of the Nine Days (even if one does not avail oneself of the dispensation to eat meat).
Citing the verse "Zion shall be redeemed with mishpat [Torah] and its returnees with tzedakah," (Isaiah 1:27) the Rebbe urged that we increase in Torah study (particularly the study of the laws of the Holy Temple) and charity during this period.