After having been allowed back into France in the year 1315 (after the expulsion in 1306 by Philip IV), the Jews were once again expelled from France by Charles IV, who thus broke the pledge made by his predecessors in 1315 that the Jews would be able to stay in France for at least 12 years.
During the Three Weeks, from 17th of Tamuz to the 9th of Av, we commemorate the conquest of Jerusalem, the destruction of the Holy Temple and the dispersion of the Jewish people.
Weddings and other joyful events are not held during this period; like mourners, we do not cut our hair, and various pleasurable activities are limited or proscribed. (The particular mourning customs vary from community to community, so consult a competent halachic authority for details.)
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.
Links:
The Three Weeks
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?