The Dow Jones Industrial Average suffered its largest one-day percentage fall in the 20th century on October 19, 1987.
That Monday became known as Black Monday after the Dow plunged 508 points, or 22.6%, in a single trading session. The percentage loss remains one of the most dramatic daily declines recorded by a major stock-market index.
The crash was international rather than purely American. Markets in Hong Kong, London, Australia, and Canada also fell sharply, with trading systems and investor behavior transmitting panic across time zones. Program trading and portfolio-insurance strategies amplified selling pressure.
Black Monday is often confused with the 1929 Wall Street Crash, whose most famous sessions occurred on October 24 and October 29. The 1987 event was more severe as a one-day percentage decline, but it did not produce the prolonged economic collapse associated with the Great Depression.