The 29 October 1929 day when U.S. stock prices plunged during the Wall Street Crash is known as Black Tuesday.
Black Tuesday followed Black Thursday, 24 October, and Black Monday, 28 October. On 29 October, roughly 16 million shares changed hands on the New York Stock Exchange, a record at the time. The Dow Jones Industrial Average fell sharply as investors rushed to sell.
The crash did not cause the entire Great Depression by itself. It exposed weaknesses that included excessive speculation, purchases made on margin, uneven income distribution, and financial fragility. Banks and businesses later failed in large numbers, while unemployment rose dramatically.
A common mix-up is treating Black Tuesday as the beginning of the crash. Stock prices had already fallen substantially from their September peak, and the panic unfolded over several trading sessions. Black Tuesday is specifically the final major sell-off day in October 1929.