The largest one-day percentage fall in the Dow before Black Monday 1987 was 12.8 percent, recorded on October 28, 1929.
That session is commonly called Black Monday of 1929. It was followed by Black Tuesday on October 29, when the Dow fell another approximately 12 percent. The paired losses made the final days of October especially destructive for investors.
The 12.8 percent decline was later surpassed dramatically by October 19, 1987, when the Dow lost 22.6 percent. The comparison is based on percentage change, not the number of index points, because index levels differed greatly between the two eras.
The 1929 decline occurred after a speculative boom supported by margin buying. Falling prices triggered forced selling and eroded confidence. Although the crash was a major turning point, the Great Depression’s full economic damage emerged through a combination of financial, monetary, banking, and international factors.