The Wall Street Crash of 1929's most famous one-day collapse, known as Black Tuesday, occurred on October 29, 1929.
On that Tuesday, heavy selling overwhelmed the New York Stock Exchange. The Dow Jones Industrial Average fell about 12%, following major losses on Black Thursday and Black Monday. The sequence shattered confidence in the market and became the defining financial event at the start of the Great Depression.
The crash did not by itself cause every feature of the Depression. Banking failures, falling industrial production, deflation, debt problems, and policy decisions also deepened the economic downturn. A common mix-up is treating Black Tuesday as the entire crash; it was one dramatic day in a broader collapse that continued into 1932.
The Dow eventually lost nearly 90% of its value from its 1929 peak to its 1932 low. It did not regain its 1929 closing high until 1954.