19 October 1987 was called Black Monday because the Dow Jones Industrial Average fell 22.6% in one session.
The decline was the largest one-day percentage fall in the Dow’s history. Stock markets in Hong Kong, Europe, Australia, and elsewhere also suffered severe losses, making the event an international crash rather than an isolated American downturn.
Several factors contributed, including high valuations, rising interest rates, trade tensions, and portfolio-insurance strategies that automatically sold futures as prices declined. Those automated sales intensified the downward movement, although historians and economists continue to debate the precise weighting of each cause.
Black Monday is sometimes confused with the 1929 crash dates, especially Black Tuesday on 29 October 1929. The 1987 crash did not lead to a depression comparable to the 1930s. Central banks supplied liquidity, and the Dow eventually recovered its pre-crash closing level.