October 19, 1987, is known as Black Monday because the Dow Jones Industrial Average suffered its largest one-day percentage fall on that date.
The Dow dropped 508 points, or 22.61%, during the session. Stock markets in other countries also fell sharply, making the event an international market crash rather than an isolated U.S. decline.
Several forces contributed, including overvalued shares, rising interest rates, trade tensions, and widespread computerized portfolio-insurance strategies. Automatic selling amplified the decline as prices weakened.
Black Monday is sometimes confused with Black Thursday, October 24, 1929, an important day in the Wall Street Crash. The two events occurred nearly six decades apart and had different immediate causes.