Black Tuesday was October 29, 1929, when the New York Stock Exchange suffered a historic sell-off.
The Dow Jones Industrial Average fell about 12% that day, with roughly 16 million shares changing hands. The selling followed Black Thursday on October 24 and Black Monday on October 28, creating a sequence of panic-driven declines.
Black Tuesday did not cause the entire Great Depression by itself. The crash exposed severe weaknesses, including buying stocks on margin, overproduction, weak bank regulation, and falling consumer demand. Economic contraction continued for years afterward.
A common mix-up is treating October 29 as the exact beginning of the crash. The market had already dropped sharply before that date, but Black Tuesday became the best-known symbol of the 1929 collapse.