“When Av begins, we diminish [our] rejoicing” (Talmud, Taanit 26b).
On the 1st of Av, “The Three Weeks” mourning period over the destruction of the Holy Temple–which began 13 days earlier on Tammuz 17–enters an intensified stage.
During “The Nine Days” from Av 1st to the Ninth of Av, a heightened degree of mourning is observed, including abstention from meat and wine, music, bathing for pleasure, and other joyous and enjoyable 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 a mitzvah) such as a Bris (circumcision), or a “Siyum” celebrating the completion of a course of Torah study (i.e., a complete Talmudic tractate).
The Lubavitcher Rebbe 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 (Isaiah 1:27) “Zion shall be redeemed with mishpat [Torah] and its returnees with Tzedakah,” the Rebbe urged that we increase in Torah study (particularly the study of the laws of the Holy Temple) and charity during this period.