Black Friday

Black Friday – the day after Thanksgiving and the most profitable shopping day of the year in the US, got its name from the old accounting practice of using red ink for debt and black ink for profit.

DC USA Black Friday By Gridprop at English Wikipedia 

The original "Black Friday" was Friday, September 24, 1869 when two notoriously ruthless Wall Street financiers, Jay Gould and Jim Fisk, worked together to corner the market on gold, buying as much of it as they could and driving up the price. On that Friday in September, the conspiracy finally unraveled, sending the stock market into free-fall and bankrupting everyone from Wall Street barons to farmers.

Police officers in Philadelphia were first to link Black Friday to the post-Thanksgiving period in the 1950s. Large crowds of tourists and shoppers came to the city the day after Thanksgiving for the Army-Navy football game, creating chaos, traffic jams and shoplifting opportunities.  Not only would Philly cops not be able to take the day off, but they would have to work extra-long shifts dealing with the additional crowds and traffic.

Use of the "Black Friday" phrase spread slowly, first appearing in The New York Times on November 29, 1975, in which it still refers specifically to "the busiest shopping and traffic day of the year" in Philadelphia.  By the late 1980s, the term was commonly known across the nation and retailers soon linked it to their post-Thanksgiving sales.

Sources History Daily Telegraph