The stock market lost a quarter of its value in four trading days in 1929

Between October 24 and October 29, 1929 — 'Black Thursday' through 'Black Tuesday' — the New York Stock Exchange lost about a quarter of its value as panic selling overwhelmed buyers. The crash didn't cause the Great Depression by itself, but it shattered confidence and triggered a wave of bank failures that deepened it for a decade.

— Wikipedia contributors, Wall Street Crash of 1929 — October 1929