Tevet 5 is celebrated as a day of rejoicing in the Chabad-Lubavitch community. On this date in 1987, U.S. Federal Court issued a decision in favor of Agudas Chassidei Chabad ("Union of Chabad Chassidim") regarding the ownership of the priceless library of the 6th Rebbe of Chabad-Lubavitch, Rabbi Yosef Yitzchak Schneersohn. The ruling was based on the idea that a Rebbe is not a private individual but a communal figure synonymous with the body of Chassidim. The Lubavitcher Rebbe (Rabbi Yoseph Yitzchak's son-in-law and successor) urged that the occasion be marked with time devoted to study from Torah books ("sefarim") as well as the acquisition of new Torah books.

Links:
Books with Souls
The Rebbe's Library
Chabad.org online library