Tes Kislev is both the birthday and day of passing of the Mitteler Rebbe, Rabbi DovBer of Lubavitch, son of the Alter Rebbe and the second Lubavitcher Rebbe.
The Mitteler Rebbe was known for his unique style of "broadening rivers" -- his teaching were the intellectual rivers to his father's wellspring, lending breadth and depth to the principles set down by the Alter Rebbe.
Born in Liozna, White Russia in 1773, Rabbi DovBer was named after Rabbi Schneur Zalman's mentor and teacher, Rabbi DovBer of Mezeritch, who had passed away on Yud Tes Kislev of the previous year.
Rabbi DovBer assumed the leadership of Chabad upon his father's passing in 1812. In 1813 he settled in the town of Lubavitch, which was to serve as the movement's headquarters for the next 102 years. In 1826, he was arrested on charges that his teachings threatened the imperial authority of the Czar, but was subsequently exonerated.
The Mitteler Rebbe passed away on his 54th birthday in 1827, a day before the first anniversary of his liberation.
The date of his release, Kislev 10, is celebrated amongst Chabad Chassidim as a festival of liberation. Tachnun is omitted, farbrengens are held, and the Mitteler Rebbe’s teachings are studied.