The largest one-day percentage fall in the Dow Jones Industrial Average during the 1929 crash occurred on October 28, 1929.
The Dow fell 12.82% on that Monday, which became known as Black Monday. The following day, October 29, brought another historic decline of 11.73% and became known as Black Tuesday. These dates are often blended together in popular accounts.
The crash followed years of rising share prices, heavy borrowing to buy stocks, and speculative enthusiasm. Falling prices forced investors to sell, while margin calls accelerated the downward movement. The crash was a major event in the beginning of the Great Depression, although economists also identify banking failures, deflation, and international economic problems as important causes.
The 1929 collapse was not a single-day event. Prices continued to weaken after October, and the Dow eventually reached its bear-market low in July 1932.