October 29, 1929, was called Black Tuesday during the Wall Street Crash of 1929.
The Dow Jones Industrial Average fell 12.8% on that day as roughly 16 million shares changed hands, then a record trading volume. The collapse followed Black Thursday on October 24 and Black Monday on October 28, when heavy selling had already shaken confidence.
The crash did not single-handedly cause the Great Depression. Economic weakness, bank failures, debt, falling demand, and policy responses all contributed to the prolonged downturn. However, the crash became its most famous financial symbol.
A common mix-up is treating Black Tuesday as the entire crash. It was one especially dramatic session in a broader decline that began in September 1929 and continued into 1932, when the Dow reached its low for the cycle.