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
Where the Infinite Light is hidden
—there you see a world.
If you could see the light that generates this world
You would not see any world at all.
And yet the world you see
is nothing more than G-d's own hand hiding His light.
That is all there is to your world:
G-d hiding behind Himself.
Why?
So that you will search for Him
and find Him in this world of yours.