After nearly a month of fierce fighting inside Jerusalem (see "Today in Jewish History" for Tammuz 9), the armies of King Nebuchadnezzar of Babylonia broke through into the Temple compound, where they feasted and vandalized until the afternoon of Av 9, when they set the Holy Temple aflame.
Links: The Destruction of the Holy Temple
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.
Links:
Nine Days laws and customs
Daily live siyum broadcasts
Learn about the Holy Temple in Jerusalem
Our souls are in exile within our bodies. Our people is in exile within a foreign world.
And so there are two things we must know:
That this is not our place.
And that hidden treasure lies buried here; for G‑d dwells in darkness.
If we remember only that this is not our place, we may remain strong, we may even shine in the darkness. But what profit will there be from our exile?
And if we remember only that treasure lies here, we will begin to believe that this is our place, and if so, of what use is the treasure?