The 1929 Wall Street Crash's biggest one-day percentage loss occurred on October 28, 1929, known as Black Monday.
The Dow Jones Industrial Average fell 12.82% that day, followed by another exceptionally severe decline on October 29, known as Black Tuesday. The selling came after weeks of weakening confidence in heavily speculative U.S. stocks.
The crash did not by itself create the Great Depression, but it became its most famous financial symbol. Banking failures, falling industrial production, deflation, and unemployment deepened during the years that followed. The Dow eventually lost almost 89% from its September 1929 peak to its July 1932 low.
A common mix-up is to identify October 24, Black Thursday, as the largest percentage fall. That day was the first major panic session, but October 28 produced the sharper percentage decline. October 29 had heavier trading and is often remembered as the crash's defining day.