Lots of cars, lots of traffic, lots of chaos. Sound familiar?
So at some point in the 1950s or 1960s -- some put the date exactly at 1966 -- the Philadelphia Police Department started to refer to the day after Thanksgiving as “Black Friday,” with the unrealistic hope that people would find the whole shebang distasteful and opt out of the collective consumer madness. At a minimum, it was a derisive way to describe an unpleasant day in the life of a Philly cop.
From wiki:
The day's name originated in Philadelphia, where it originally was used to describe the heavy and disruptive pedestrian and vehicle traffic which would occur on the day after Thanksgiving.[6][7] Use of the term started before 1961 and began to see broader use outside Philadelphia around 1975.