The largest one-day percentage loss in the Dow before the 1987 crash was 12.8%, recorded on October 28, 1929.
This fall occurred on Black Tuesday’s immediate predecessor, commonly called Black Monday in accounts of the 1929 Wall Street Crash. The Dow had already plunged on Black Thursday, October 24, and selling intensified as confidence collapsed at the end of October.
The 12.8% decline is a percentage measurement, not a point total. Index levels were much lower in 1929, so comparing raw points across decades can mislead. Percentage changes provide a more meaningful comparison between crashes separated by many years.
The figure was surpassed dramatically on October 19, 1987, when the Dow lost 22.6%. That later event is why Black Monday is most commonly associated with 1987, while October 28, 1929, is usually described as Black Monday in historical accounts of the earlier crash.