During the 1929 Wall Street crash, the Dow Jones Industrial Average lost about 23% over Black Monday and Black Tuesday combined.
On October 28, 1929, the Dow fell 12.8%, followed by a further 11.7% decline on October 29. Together, those losses amounted to roughly 23% in two trading sessions. The consecutive falls were among the most dramatic episodes of the crash.
The dates are often confused with Black Thursday, October 24, when heavy selling first attracted widespread attention. The market did not reach its ultimate low immediately: the Dow continued falling and bottomed in July 1932, almost 90% below its September 1929 peak.
The 1929 crash was one part of a broader economic collapse that became the Great Depression. Historians debate how much responsibility belongs to speculation, margin lending, banking weakness, monetary policy, and international economic conditions.