Black Monday refers to Monday, October 19, 1987, when stock markets around the world crashed. The crash began in Hong Kong, spread west through international time zones to Europe, hitting the United States after other markets had already declined by a significant margin. The Black Monday decline was the largest one-day percentage decline in stock market history. Other large declines have occurred after periods of market closure, such as, in the USA, on Monday, September 17, 2001, the first day that the US market was open following the September 11, 2001 attacks. Crash is also referred to as Black Tuesday because of the time zone difference.