Which 1987 trading session produced the largest one-day percentage loss in the S&P 500?

October 19, 1987, produced the largest one-day percentage loss in the S&P 500, a fall of 20.47 percent. The session is widely known as Black Monday and affected markets worldwide.

Program trading, portfolio insurance, overvalued shares, rising interest rates and international market tensions all contributed to the selling. Automated strategies amplified the decline as prices fell, while liquidity on the exchange became limited. Unlike a normal correction, the market moved extraordinarily far in a single day.

The Dow Jones Industrial Average dropped 22.6 percent, its largest daily percentage decline. Trading reforms followed, including circuit breakers designed to pause markets during extreme moves. The crash did not lead to a prolonged economic depression in the United States, and stocks recovered much of the loss within about two years.