Today in Jewish History: 31 Years Since the Victory in Chabad Library Trial
On the 5th of Tevet (Hei Teves) in 1987, the extensive library of the sixth Rebbe, Rabbi Yosef Yitzchak Schneerson, was declared as belonging to Agudas Chasidei Chabad Lubavitch. The U.S. Federal court issued this ruling after a lengthy, highly-publicized trial. The anniversary of this event is marked annually, by purchasing new sefarim (Jewish books) and studying them.
The Kehot Publication Society hosts a mass annual Hei Teves sale for Jewish books, see link here: http://store.kehotonline.com/