The day after Thanksgiving has been regarded as the beginning of America's Christmas shopping season since the 1950s, although the term "Black Friday" didn't become widely used until more recent years.

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. Police officers in the city weren't able to take the day off work and instead had to work long shifts to control the carnage, coining the phrase Black Friday to refer to it.

Black Friday represents the point in the year when retailers begin to turn a profit, thus going from being "in the red" to being "in the black".

The most violence seems to occur at Walmart (57.1 percent of Black Friday incidents) leading to the Twitter hashtag each year #Walmartfights.

As the name spread throughout Philadelphia in the 1950s, some of the city's merchants disliked the negative connotations and unsuccessfully tried to change it to "Big Friday".

The department store Macy's was the first to advertise post-Thanksgiving shopping in 1924, during their Thanksgiving Day Parade in New York.

An estimated 52% of shoppers purchase something on sale that they later regret.

Professional line sitters make up to $35 an hour, and Black Friday is their busiest day of the year.

The Monday after Thanksgiving was first given the name "Cyber Monday" in 2005 by Shop.org and has since become nearly as important to retailers as Black Friday.

Use of the 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.