The 1987 market event that became the largest one-day percentage fall in the Dow Jones Industrial Average was Black Monday.
Black Monday occurred on October 19, 1987. The Dow Jones Industrial Average dropped 22.6 percent in a single trading session, the largest one-day percentage decline in the index’s history. Major markets in Europe, Asia, and elsewhere also suffered severe falls.
Several factors contributed, including high valuations, concerns about interest rates and trade deficits, and selling strategies that used computer-based portfolio insurance. As prices declined, those strategies could generate additional sell orders and intensify the fall.
The crash did not produce a Great Depression-style economic collapse. Central banks, especially the U.S. Federal Reserve under Alan Greenspan, supplied reassurance and liquidity. Regulators later developed market-wide circuit breakers designed to pause trading during exceptionally rapid declines.