On the 2nd and 3rd of Av in the year 5702 from creation (1942 CE), more than 13,000 Jews were rounded up by French police and interred in the Vel' d'Hiv, an indoor bicycle stadium in the center of Paris. They were later transported to Auschwitz to be killed. Within days, the Vel' d'Hiv was cleaned up and ready for recreation.
In the summer of 1929, R. Yosef Yitzchak Schneersohn, sixth Rebbe of Chabad, visited the Land of Israel (the only Chabad Rebbe to do so—see link below). The stated purpose of the trip was to pray at the gravesites of the righteous individuals interred there. Among the cities he visited during his two-week-long stay were Jerusalem, Safed, Meron, Tiberias, Hebron, and Tel Aviv.
The Rebbe departed the Holy Land two days before the Arab riots of 1929, in which scores of Jews were massacred in Hebron and Jerusalem (see entry for 17 Av).
Links: Cause and Effect, Why Didn’t the Rebbe Ever Visit Israel?
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
In Torah, we mirror on earth that which G‑d performs on every plane of reality.
If so, since the Torah prohibits dislocating even a single stone of the Holy Temple in Jerusalem, how could it be that G‑d brought the entire structure to ruins?
For it would certainly be absurd to imagine that the Assyrians or the Romans had the power to set fire to G-d’s house.
It must be that this was not an act of destruction. Rather, it was the initial phase of a much greater construction, one that would be eternally indestructible.
And for that to occur, the Temple had to be temporarily leveled to its foundations and G-d’s people had to be scattered to the furthest reaches of human habitation.
Why? Because as long as there is any place in this world that considers itself outside the realm of holiness, there remains a place for the destruction of G‑d’s Temple.
But in our exile, we meet face to face all that considers itself foreign to the divine. We grasp its reins, extract its poison, and channel its power.
This third and ultimate Temple, then, will be built of the outside turned inward, of darkness taught to shine, of the other converted to the One, of the most sinister enemy transformed to a faithful ally.
No opposition will remain in the universe. And so it will last forever.
Then we will see that in truth, there was never any destruction. There was only rebuilding, growth, and eternal, deep love.