The S&P 500 lost 20.47 percent on Black Monday, October 19, 1987.
Black Monday was the largest one-day percentage decline in the history of the S&P 500. The Dow Jones Industrial Average also fell dramatically, while markets in several countries experienced similarly severe losses. The speed and international reach of the decline made it a defining event in modern financial history.
Several factors contributed, including high valuations, rising interest-rate concerns, trade tensions, and computerized portfolio-insurance strategies. Those automated strategies could generate additional sell orders as prices dropped, intensifying the downward movement.
The crash did not produce a depression like the 1929 collapse. The Federal Reserve, led by Chairman Alan Greenspan, signaled its readiness to provide liquidity to the financial system. Market structures were later changed, including the introduction of circuit breakers designed to pause trading during extreme declines.