Black "Day" has been a slang term for any day of tragedy or economic stress since the 17th century. The first widespread recorded use of "Black Friday" was to describe a crash of the U.S. gold market in 1869.
https://en.m.wikipedia.org/wiki/Black_Friday_(1869)
The use of Black Friday to describe the day after Thanksgiving began in Philadelphia in the early 1950s, used by the local please to describe the crowds and traffic congestion.
https://www.history.com/news/black-friday-thanksgiving-origins-history
Kudos to all. I appreciate how things were discussed reasonably and respectfully, and how when an error was presented with evidence, the mistake was owned without need for incident.
https://en.m.wikipedia.org/wiki/Black_Friday_(1869)
The use of Black Friday to describe the day after Thanksgiving began in Philadelphia in the early 1950s, used by the local please to describe the crowds and traffic congestion.
https://www.history.com/news/black-friday-thanksgiving-origins-history
Black Friday (shopping) has it's own separate listing which corroborates the second link @felixo77 used. https://en.wikipedia.org/wiki/Black_Friday_(shopping)
Interestingly, the US is no longer the only country to use "Black Friday" as a sales day.
Thanks for the fact checking and sources.