Black Friday

A Friday falling on the 13th day of the month (and therefore doubly ill-omened).

(possibly obsolete) Good Friday.

Any Friday actually darkened by catastrophe, or the anniversary thereof.

(Internet slang, poker, historical) The conclusion of United States v. Scheinberg in 15 April 2011, after which major online poker sites stopped offering real money play to their United States customers.

(US, Canada, business, retailing) The day after US Thanksgiving Day, generally regarded as the first day of the Christmas season, and the busiest shopping day of the year. Observed in the US and, more recently, Canada.

(by extension) The sales period involving heavy price reductions immediately following US Thanksgiving Day, from Friday (the original Black Friday) through Monday (Cyber Monday).