The October 29, 1929 US stock-market plunge was called Black Tuesday.
Black Tuesday was the most famous single day of the Wall Street Crash of 1929. On that date, about 16 million shares changed hands on the New York Stock Exchange, a record at the time, and the Dow Jones Industrial Average fell about 12%. The selling followed Black Thursday on October 24 and Black Monday on October 28.
The crash did not by itself create the entire Great Depression, but it severely damaged confidence, wealth, and business investment. Banks, companies, and households were affected as falling asset prices interacted with debt, bank failures, and reduced spending.
A common mix-up is treating Black Tuesday as the whole crash. The broader Wall Street Crash unfolded over several days in October 1929, while Black Tuesday specifically refers to October 29.