'Black Friday' is the name that the Philadelphia Police Department gave to the Friday following Thanksgiving Day.

It is not a term of endearment for them. 'Black Friday' officially opens the Christmas shopping season in center city, and it usually brings massive traffic jams and over-crowded sidewalks as the downtown stores are mobbed from opening to closing.

Source: http://useconomy.about.com/od/demand/f/Black_Friday_Name.htm