When the crusaders captured Jerusalem during the First Crusade, the Jews of Jerusalem fled into a synagogue. The crusaders then set flame to the synagogue, burning alive all the Jewish men, women, and children who had taken refuge there. All Jews were barred from living in the city of Jerusalem for the following 88 years.
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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.
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The Three Weeks
From a letter:
Even if all your complaints about your spouse are well-founded and valid—show her your love, nevertheless. Show her unconditional love.
The sages taught that all our exile is due to the sin of senseless, unwarranted hatred.
When each one of us will start with unwarranted, unconditional love in our own domain, from there it will spread to all else that we do, and from there to the entire world, speedily in our days, amen.