The S&P 500 bottomed during the COVID-19 crash in March 2020.
Its closing low came on March 23, 2020, at 2,237.40. From the record closing high of 3,386.15 on February 19, the index had fallen roughly 34% in just over a month. The decline reflected rapidly spreading COVID-19 infections, emergency lockdowns, collapsing economic expectations, and turmoil in credit and commodity markets.
March contained several extraordinary trading sessions. On March 9, March 12, and March 16, U.S. markets triggered circuit breakers after sharp declines. March 16 was especially severe: the S&P 500 fell nearly 12%, while the Dow suffered its largest percentage loss since 1987 at that point.
The March 23 low marked the trough of the crash, not the end of all market uncertainty. Massive Federal Reserve support, fiscal stimulus, and improving expectations helped launch a powerful rebound. The S&P 500 later recovered its pre-crash closing high in August 2020. “Bottom” here means the index's closing trough; intraday lows and other market measures can differ slightly.