The largest single-day percentage fall in the Dow Jones Industrial Average before the 1987 crash was 12.82% on 28 October 1929.
The decline occurred during the four-day sequence surrounding the Wall Street Crash of 1929. On Black Monday, 28 October, the Dow fell 38.33 points, or 12.82%, as selling intensified after the earlier collapse on Black Thursday.
The next day, Black Tuesday, brought another severe loss of 11.73%. These declines were later surpassed dramatically by Black Monday in 1987, when the Dow dropped 22.6% in a single session.
The wording matters because the 1929 figure is a percentage decline in the Dow, not the largest point loss. Point comparisons across eras can mislead because the index level changes over time, so percentage measures are normally used to compare crashes historically.