Definition from Wiktionary, the free dictionary
Etymology 1 
- A Friday falling on the 13th day of the month (and therefore doubly ill-omened).
- Any Friday actually darkened by catastrophe.
- The anniversary of a number of catastrophic or unfortunate historical events originally occurring on a Friday.
- (obsolete?) Good Friday
Coordinate terms 
Etymology 2 
Proper noun 
- (US, business, retailing) The day after Thanksgiving Day, generally regarded as the first day of the Christmas season, and the busiest shopping day of the year. (Although this usage may also very possibly originate in Etymology 1)