It is erroneously believed by many that the term Black Friday derives from the concept that businesses operate at a financial loss, or are “in the red,” until the day after Thanksgiving when massive sales finally allow them to turn a profit, or put them “in the black.” However, a more accurate explanation of the term dates back to the early 1960s, when police officers in Philadelphia began using the phrase “Black Friday” to describe the chaotic crowds that resulted when large numbers of suburban tourists came into the city to begin their holiday shopping.

Source: Encyclopedia Britannica