Proper noun
(business, historical, economics, finance) The day when the New York Stock Exchange crashed before the Great Depression; Thursday, October 24th, 1929.
(business, historical, economics, finance) March 12, 2020, as part of the 2020 stock market crash; the day on which United States stock markets saw their biggest single-day percentage drop since 1987.
Source: en.wiktionary.org