The S&P 500’s largest single-day percentage fall during the 2020 stock-market crash was about 12.0% on 16 March 2020.
That session came during the rapid market sell-off associated with the COVID-19 pandemic. Investors reacted to worsening public-health news, travel restrictions, business shutdowns, and fears of a severe global recession. The S&P 500 fell 11.98%, commonly rounded to 12.0%, while other major US indexes also recorded historic declines.
The market had already entered a sharp downturn in February and early March. On 9 March, 12 March, 16 March, and 18 March, US exchanges experienced extraordinary volatility, with circuit breakers triggered on several occasions. The Federal Reserve and other authorities introduced emergency measures as the economic outlook deteriorated.
The 12.0% number is a one-day closing-to-closing change, not the complete crash. From its February 2020 record close to its March low, the S&P 500 lost roughly one-third before beginning a powerful recovery.