Black Tuesday was the name of October 29, 1929, when the Dow Jones Industrial Average fell 11.7%.
The day followed Black Thursday, October 24, and Black Monday, October 28, as panic selling overwhelmed the New York Stock Exchange. About 16 million shares changed hands on October 29, a record at the time, while prices fell across many heavily traded companies.
Black Tuesday did not cause the entire Great Depression by itself. The crash damaged confidence and wealth, but the depression also involved bank failures, falling demand, deflation, international debt problems, and policy decisions. The Dow continued declining after 1929 and eventually reached its low in July 1932.
A common mix-up is treating Black Tuesday as the only crash day. It was the most famous session of a broader collapse that unfolded over several days and had consequences lasting years.