The biggest one-day percentage decline of the Dow appeared on Black Monday 1987, and the average index fell by 22.6%. The biggest point drop in a single day occurred on September 17, 2006, that is, the day after the 9 1 1 attack. At that time, the Dow fell 684.8 1 point, down 7. 1% year-on-year. At the end of the week, the Dow Jones Industrial Average fell 1369.70 points, down 14.3% year-on-year.