The Dow Jones Industrial Average suffered its famous 22.6% plunge on October 19, 1987, during the Black Monday crash.
The fall was the largest one-day percentage decline in the Dow's history. It occurred during a worldwide market rout: Hong Kong fell first, European markets followed, and U.S. shares then plunged. The crash was not confined to one country or one exchange.
Analysts have linked the sell-off to high valuations, rising interest rates, trade tensions, and computerized portfolio-insurance strategies that accelerated selling. The exact combination remains debated, but the date and Dow decline are firmly established.
Black Monday is sometimes confused with Black Tuesday in 1929. Black Tuesday was October 29, 1929; Black Monday in this question refers to October 19, 1987, a separate global market shock.