The catastrophic U.S. stock-market sell-off on October 29, 1929, is known as Black Tuesday.
Black Tuesday followed Black Thursday, October 24, and Black Monday, October 28, during the Wall Street Crash of 1929. On October 29, investors traded roughly 16 million shares on the New York Stock Exchange, a record at the time, while prices fell sharply. The crash did not create every problem of the Great Depression, but it intensified a financial collapse that was already spreading through banks, businesses, and households.
The name refers specifically to the date, not to the entire multi-day crash. The broader event is usually called the Wall Street Crash of 1929, while Black Tuesday identifies its most famous trading day. A common mix-up is treating the crash as an immediate one-day start to the Great Depression; the economic downturn developed through several years of falling production, bank failures, and unemployment.