Tuesday, October 29, 1929, was called Black Tuesday because the Dow Jones Industrial Average fell 12.8% during the Wall Street Crash.
The decline followed Black Thursday, October 24, and Black Monday, October 28. On Black Tuesday, roughly 16 million shares changed hands, overwhelming the market's trading systems and intensifying panic among investors. The crash did not by itself create the entire Great Depression, but it became its most famous financial symbol.
The Dow's percentage loss on Black Tuesday was smaller than its 22.6% fall on October 19, 1987, often called Black Monday. Black Tuesday is specifically the 1929 date, while Black Monday can refer to several different market disasters. The Dow eventually lost almost 89% from its September 1929 peak to its July 1932 low.