The S&P 500 fell about 20.5% on October 19, 1987, in the global Black Monday crash.
The S&P 500 measures large U.S. companies and is weighted by market capitalization, so its daily movement represented a broad market collapse rather than a fall in only a few shares. On Black Monday, the index lost roughly one-fifth of its value in a single session, while the Dow fell 22.6%.
The crash occurred after several years of strong gains and amid anxiety about interest rates, trade, and economic growth. Portfolio insurance strategies and automated selling were widely cited as mechanisms that increased downward pressure. Trading systems and market rules were not designed for such an extraordinary volume of orders.
The Federal Reserve responded by publicly affirming its readiness to provide liquidity to the financial system. The market recovered much of the decline over time, and the crash did not produce a depression. The S&P 500 percentage is distinct from the Dow's larger headline loss.