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
Naturally, we think of the Jewish people as a conglomerate of many Jews. But the Baal Shem Tov saw the Jewish people as a single, indivisible whole.
Think of a geometrical point. A point is indivisible, but not because it is too hard, too big, or too small to cut up. A point simply has no area to be divided. That’s what makes it a point.
And yet, from a point you can extend infinite lines radiating in infinite dimensions.
In a somewhat similar way, but far beyond, all Jews are one Jew. Which means that in any one Jew, you will find all of us—just from a different angle.
So that whatever happens to any one of us instantaneously happens to the entire Jewish people. Not by some ripple effect or resonance. But because any one sample of the whole is the whole and the whole is one.
And so, the Baal Shem Tov taught, when the light of any one Jewish soul breaks free, the entire nation is redeemed along with it.
And accordingly, the Rebbe wrote, the ultimate exodus of our entire people is also a personal, intimate liberation for every Jew.