You've probably heard that "Black Friday" refers to stores going into profitability, from the use of black ink to signify a positive number on a profit-and-loss statement (as opposed to red ink for negative).
But Kevin Drum explains that this is actually pretty clearly not true, no matter how often the media repeat this folk etymology. The term originated in Philadelphia in the 1950s when boisterous football fans in town for the Army-Navy game would spend Friday crowding into downtown stores and causing annoyance for store employees. It was likely influenced by the use of "Black ___day" to refer to various disasters, such as "Black Tuesday" for the 1929 stock market crash.
Apparently the term "Black Friday" was not generally used across the U.S. even as recently as the mid-1980s. Now, of course, it's commonplace, and it has apparently even spread to the UK despite the fact that Thanksgiving isn't a British holiday. See Drum's article for more.