The S&P 500 fell about 57% from its 2007 peak to its March 2009 low during the 2008 financial crisis.
The index peaked at 1,565.15 on October 9, 2007, and reached a bear-market low of 676.53 on March 9, 2009. The fall reflected the collapse of the U.S. housing boom, mortgage defaults, losses on complex credit products, and a worldwide contraction in lending and economic activity.
The decline was not confined to financial shares. Banks, industrial companies, technology firms, and consumer businesses were all affected as credit tightened and investors anticipated recession. The crisis also produced major government rescue programs and central-bank interventions.
The 57% figure describes the peak-to-trough decline in the S&P 500 during that bear market. It is different from the percentage loss on any single day, which was much smaller.