During the 1929 Wall Street crash, the Dow Jones Industrial Average suffered its largest percentage loss of that crash week on Black Monday, October 28.
The Dow fell 12.82 percent on Black Monday and then dropped another 11.73 percent on Black Tuesday, October 29. These back-to-back declines followed weeks of heavy selling and helped destroy confidence in the highly speculative market of the late 1920s. Investors had frequently bought shares on margin, borrowing money to amplify their positions.
Black Tuesday is often remembered as the single defining date of the crash, but the collapse unfolded over several trading sessions. The crash did not by itself cause every part of the Great Depression; banking failures, falling demand, monetary-policy problems, and international debt difficulties also mattered. The Dow eventually recovered, but it did not regain its September 1929 closing high until 1954.