The Dow Jones Industrial Average fell 12.82% on Black Monday, October 28, 1929. It was one of the most severe daily declines during the Wall Street Crash of 1929.
The next trading day, Black Tuesday, brought another collapse of 11.73%. Together, the two sessions shattered confidence in the U.S. stock market and intensified public fear about banks, businesses, and personal savings.
The crash followed years of speculative buying, widespread use of borrowed money, uneven economic growth, and high stock valuations. It did not by itself cause the entire Great Depression, but it became its most famous financial symbol. The Dow eventually lost almost 89% from its September 1929 peak to its 1932 low.
A common mix-up is confusing Black Monday in 1929 with Black Monday in 1987. The 1929 date was October 28; the better-known 1987 crash occurred on October 19 and involved a much larger one-day percentage fall.