Stocks go down. On September 17, 2001, the Dow Jones Industrial Average dropped 685 points from its opening level of 9605. This was the biggest one-day decline ever and occurred on the first day the New York Stock Exchange was open following the attacks on September 11, 2001. By what percentage did the Dow drop that day? On October 28, 1929, the Dow Jones Industrial Average dropped 38 points from its opening level of 261. By what percentage did the Dow drop that day? This was the second-biggest one-day percentage drop ever.