The 1929 U.S. stock-market crash day when the Dow lost nearly 12% is commonly called Black Tuesday.
Black Tuesday was October 29, 1929. The Dow Jones Industrial Average fell 30.57 points, a decline of about 11.7%, amid extraordinary trading volume. It followed Black Thursday, October 24, and Black Monday, October 28, two earlier panic sessions in the same collapse.
The crash ended the speculative boom of the 1920s, but it did not by itself cause every aspect of the Great Depression. Banking failures, falling demand, deflation, protectionist policies, and monetary problems deepened the economic downturn. The Dow continued falling after October 29 and did not reach its ultimate low until July 1932.
Black Tuesday is therefore the name of a specific trading day, not the entire depression. A common mix-up is treating it as the first day of the crash; Black Thursday came five days earlier.