The S&P 500 fell about 34% from peak to trough during the COVID-19 market crash in 2020.
The index reached a record closing high on February 19, 2020, before collapsing as the coronavirus spread internationally and investors assessed the likely economic effects of lockdowns, travel restrictions, business closures, and disrupted supply chains. Its bear-market decline reached a low on March 23.
The Federal Reserve, Congress, and governments around the world introduced emergency monetary and fiscal measures. The S&P 500 then rebounded rapidly, although the economic recovery was uneven and many sectors remained under severe pressure.
The S&P 500 tracks 500 large U.S. companies and is capitalization-weighted, so its performance is not identical to that of the Dow or Nasdaq Composite. The 34% figure refers to the approximate peak-to-trough fall, not a single trading day.