In Chabad Chassidut, the 18th of Elul (Chai Elul) is called, “the birthday of the two great luminaries”, since not only was the Baal Shem Tov born on the 18th of Elul (in the year 5458 / 1698), but also the Alter Rebbe, author of the Tanya (in the year 5505 / 1745). For this reason, the Rebbe the Tzemach Tzedek said that the 18th of Elul is a threefold festival: the birthday of the Baal Shem Tov, the birthday of the Alter Rebbe, and the beginning of the new year’s spiritual work.
In the early generations, Chai Elul was marked secretly. The Previous Rebbe relates that the 18th of Elul was one of the “hidden festivals” which the Rebbes observed privately. The Rebbes would reveal the significance of the day to their sons only when they reached the age of fifteen. The one who publicized the significance of Chai Elul and made it common knowledge was the Previous Rebbe.
And thus the Previous Rebbe crowned this day: “The 18th of Elul is the designated day for joy and blessing — for Chassidim in general, and for Chabad Chassidim in particular.”
