The Dow Jones Industrial Average’s highest one-day percentage loss during the 1929 crash was 12.8% on October 28, 1929.
That fall occurred on Black Monday, one day before the better-known Black Tuesday. The Dow dropped 38.33 points, from 299.47 to 261.14, which represented a decline of roughly 12.8%. The next day brought another dramatic fall of about 11.7%, although the percentage loss was smaller than on October 28 because the index had already dropped.
People sometimes identify Black Tuesday as the largest percentage decline of the 1929 crash, but Black Monday was worse by that measure. The crash developed after years of rising share prices, widespread speculation, and purchases made with borrowed money. Once prices weakened, margin calls forced more selling. The Dow’s losses were severe, but the broader economic catastrophe unfolded over years rather than in a single session. The index eventually lost almost 89% from its September 1929 peak to its July 1932 low.