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“Chai Elul” (Hebrew for “the 18th of Elul,” also meaning “the Vitality of Elul”) is celebrated by the Chassidic community as the birthday of the “two great luminaries” – Rabbi Israel Baal Shem Tov, the founder of Chassidism; and Rabbi Schneur Zalman of Liadi (the Alter Rebbe), the founder of Chabad.
It’s customary to wish each other “Gut Yom Tov!” and conduct joyous gatherings – farbrengens, or itvaaduyos (in Hebrew).