Black Tuesday, the most famous late-stage sell-off of the 1929 Wall Street Crash, occurred on October 29, 1929.
On that day, investors traded roughly 16 million shares on the New York Stock Exchange, and the Dow Jones Industrial Average fell about 12%. The market had already suffered major declines on Black Thursday, October 24, and Black Monday, October 28. Black Tuesday became the best-known date because selling was exceptionally intense and public confidence collapsed.
The crash did not single-handedly cause the Great Depression. It was followed by bank failures, reduced lending, falling production, unemployment, and international economic problems. The Dow continued declining after October 29 and reached its eventual bear-market low in 1932. Therefore, Black Tuesday is a key crash date, not the date when the entire economic depression began or ended.