The largest one-day percentage loss in the S&P 500 during the 2008 financial crisis was about 9.0%, on October 15, 2008.
The decline came during an exceptionally unstable period after the failure of Lehman Brothers, the rescue of major financial institutions, and growing fear that credit markets and the global economy were freezing. Investors sold shares across banking, industrial, and consumer sectors as earnings and lending prospects deteriorated.
The S&P 500 measures 500 large U.S. companies and is widely used as a broad stock-market benchmark. Its 2008 decline was part of a global market collapse linked to the U.S. housing bubble, subprime mortgage losses, excessive leverage, and failures in securitized-credit markets.
A common mix-up is confusing the 9.0% daily fall with the index's full bear-market loss. From its October 2007 peak to its March 2009 low, the S&P 500 fell roughly 57%, making the wider episode far more severe than any single session.