During the 1929 Wall Street Crash, 29 October is commonly called Black Tuesday.
On that day, about 16 million shares changed hands on the New York Stock Exchange, an enormous volume for the period. The Dow Jones Industrial Average fell roughly 12%, following another severe decline on Black Monday, 28 October.
Black Tuesday did not single-handedly create the Great Depression. The crash followed years of speculative buying, widespread use of borrowed money, weakening economic conditions, and falling confidence. It helped turn a market collapse into a major financial and economic crisis.
A common mix-up is treating Black Tuesday as the entire 1929 crash. The term specifically identifies 29 October; the broader Wall Street Crash unfolded over several weeks and included the famous 24 October sell-off known as Black Thursday.