Yud-Tes Kislev is the anniversary of the passing of R. Dovber, the Maggid of Mezritch, in 1772, and anniversary of the release from capital sentence and imprisonment of his disciple, Rabbi Shneur Zalman of Liadi, in 1798 and is celebrated as a Chassidic holiday amongst Chabad Chassidim.
Yud-Tes Kislev marks the "birth" of Chassidism: the day it was allowed to emerge from the womb of mysticism into the light of day, to grow and develop as an integral part of Torah and Jewish life.