Through the years the term "black friday" has had many different meanings. There have been movies and songs called Black Friday.
It has been used for any number of events people see as negative which happened on a friday from the stock market crash in 1929 to a crack down on online gambling sites in 2010. Most of us though associate it with the day retailers offer amazing deals to kick off the holiday shopping season.
For more on the origins of the day we all get online for in the middle of the night, or line up for hours before the stores open: click here for the History of Black Friday, the shopping day we all wait for.