Wordorigins.org

The Friday after Thanksgiving is called Black Friday. It’s the start of the holiday shopping season and is the busiest shopping day of the year. Commonly thought to be so-called because it is the day that retailers go “into the black,” in other words become profitable for the year, this is actually not the origin of the name. Like other “black” days, Black Friday is so-called because it is not a pleasant day. In this case, it is the traffic and crowds that make the day unbearable.

The term dates to at least 1975, when it appears in the New York Times on 29 November:

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.

And the Monday after Thanksgiving has been christened Cyber Monday. This day is alleged to be the busiest online shopping day of the year—with people using their internet connections at work to shop. The day, however, is not the busiest online shopping day of the year. In fact, it is nowhere near the busiest online shopping day.

Cyber Monday was coined on 19 November 2005 when Shop.org, an association of online retailers, made the claim to the New York Times that it was expecting a “substantial sales increase” on that day:

Hence the catchy Cyber Monday, so called because millions of productive Americans, fresh off a weekend at the mall, are expected to return to work and their high-speed Internet connections on Nov. 28 and spend the day buying what they liked in all those stores.