Black Friday as a term has been used in multiple contexts, going back to the nineteenth century, where it was associated with a financial crisis in 1869. The earliest uses of "Black Friday" to mean the day close to Thanksgiving come from or reference Philadelphia and refer to the heavy traffic on that day. The earliest known reference to "Black Friday" (in this sense), found by Bonnie Taylor-Blake of the American Dialect Society, refers to Black Friday 1965 and makes the Philadelphia origin explicit:
JANUARY 1966 -- "Black Friday" is the name which the Philadelphia Police Department has given to the Friday following Thanksgiving Day. It is not a term of endearment to 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.
The term Black Friday began to get wider exposure around 1975, as shown by two newspaper articles from November 29, 1975, both datelined Philadelphia. The first reference is in an article entitled "Army vs. Navy: A Dimming Splendor," in The New York Times:
Philadelphia police and bus drivers call it "Black Friday" - that day each year between Thanksgiving Day and the Army–Navy Game. It is the busiest shopping and traffic day of the year in the Bicentennial City as the Christmas list is checked off and the Eastern college football season nears conclusion. [thanks wikipedia]
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Stores advertise their day-after Thanksgiving sales as shoppers search for the best deals at South Plains Mall, Friday morning in Lubbock. Many stores, like Pac-Sun, tried to lure shoppers in with windows full of advertisements.
(Geoffrey McAllister/Lubbock Avalanche-Journal)