October 29, 1929, is commonly known as Black Tuesday, the most famous single day of the Wall Street Crash of 1929.
On that Tuesday, about 16 million shares changed hands on the New York Stock Exchange, a record at the time. The Dow Jones Industrial Average fell about 12%, following a 13% drop on Black Monday, October 28.
Black Tuesday did not begin the entire crash. Prices had already weakened after the market peaked in September 1929, and the collapse continued into 1932. The crash helped deepen the economic crisis that became the Great Depression, although historians distinguish the market collapse from the broader depression.
A common mix-up is calling October 24 Black Tuesday. October 24 was Black Thursday, while October 29 was Black Tuesday.