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
Know as clear as day and contemplate this deeply, that He who emanates all into being is He who rules and judges this world. In the heavens above and on the earth below, there is nothing else. (Deut. 4:39)
The same G‑d who transcends all time and space is the same G‑d manifest in all the forces and fields of physics.
The same G‑d from whom all existence emerges is the same G‑d who conceals that truth from every conscious being, providing space for each one to be that which it is.
The same G‑d from whom all life extends is the same G‑d who restrains that energy, with kindness, so that finite creatures can exist, to the tiniest worm hiding in the bowels of the earth.
And when you know all this, and you contemplate it well…
…that the very substance of each thing is nothing more than an articulation of divine energy, that even time and space extend from Him at will, as do the very limitations and sense of existence of each creation, that darkness is His presence as much as light, hiddenness as is revelation, matter as is energy, somethingness as is nothingness, for He is both beyond and within each place, moment and thing…
…then you will know that there really is nothing else. Nothing else but the one G‑d.
As the Baal Shem Tov would say in simple, succinct form: Divine energy is all there is, and everything is divine energy.