The S&P 500's closing low during the COVID-19 stock-market crash was 2,237.40 on March 23, 2020.
The crash followed the rapid international spread of COVID-19 and the introduction of lockdowns, travel restrictions, business closures, and other measures that threatened economic activity. Investors also reacted to an oil-price dispute between Saudi Arabia and Russia, which added stress to energy markets and corporate credit.
From its February 2020 record close to the March 23 low, the S&P 500 fell about 34%. Trading was halted several times through market-wide circuit breakers as prices moved exceptionally quickly. Central banks and governments then introduced large monetary, fiscal, and emergency-lending responses, helping markets rebound from the initial shock.
The answer is the closing low, not the intraday low. The S&P 500 later recovered its pre-crash closing high in August 2020, even though the pandemic's effects on public health, employment, and economic activity continued.