From Wikipedia:
In finance, Black Monday refers to Monday, October 19, 1987, when stock marketsaround the world crashed. The crash began in Hong Kong and spread west to Europe, hitting the United States after other markets had already sustained significant declines. The Dow Jones Industrial Average (DJIA) fell exactly 508 points to 1,738.74 (22.61%).[1] In Australia and New Zealand, the 1987 crash is also referred to as “Black Tuesday” because of the time zone difference.
The terms Black Monday and Black Tuesday are also respectively applied to October 28 and October 29, 1929, which occurred after Black Thursday on October 24, which started the Stock Market Crash of 1929.
https://en.m.wikipedia.org/wiki/Black_Monday_(1987) (Accessed 07082018)