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
Keep your trust in G‑d for yourself.
When things don’t go so well, tell yourself it is all really for the good, and rejoice in however G‑d treats you.
But when others come with their troubles, telling them they should rejoice in their afflictions can be callous and even cruel.
Cry with them, pray for them, do everything you can for them—and then you can tell them to trust in G‑d.