From Longman Dictionary of Contemporary EnglishBlack MondayˌBlack ˈMonday1Monday, 19 October 1987, the day on which shareprices on the stockexchange suddenly fell by a large amount, and many people lost a lot of money2Monday, 28 October 1929, the day on which share prices on the Stock Exchange suddenly fell by a large amount, leading to the GreatDepression of the 1930sFrom Longman Business DictionaryBlack MondayˌBlack ˈMonday noun [uncountable]1FINANCEMonday October 28,1929, when prices on the New York Stock Exchange fell by a very large amount during the wallstreetcrash2Monday October 19,1987, when prices fell by a very large amount on the New York Stock Exchange, and on other stock markets around the world3any Monday when something badhappens