http://en.wikipedia.org/wiki/Black_Monday_(1987)
<<The Black Monday decline was the largest one-day percentage decline in stock market history. Other large declines have occurred after periods of market closure, such as Saturday, December 12, 1914, when the DJIA fell 24.39%, ending the four month closure due to the outbreak of the First World War,<3> and Monday, September 17, 2001, the first day that the market was open following the September 11, 2001 attacks.
Interestingly, the DJIA was positive for the 1987 calendar year. It opened on January 2, 1987, at 1,897 points and would close on December 31st, 1987, at 1,939 points. The DJIA would not regain its August 25, 1987 closing high of 2,722 points until almost two years later.>>
http://en.wikipedia.org/wiki/Black_Friday_(1929)
<<After an amazing five-year run when the world saw the Dow Jones Industrial Average (DJIA) increase in value fivefold, prices peaked at 381.17 on September 3, 1929. The market then fell sharply for a month, losing 17% of its value on the initial leg down. Prices then recovered more than half of the losses over the next week, only to turn back down immediately afterwards. The decline then accelerated into the so-called "Black Thursday", October 24, 1929. A record number of 12.9 million shares were traded on that day. At 1 p.m. on Friday, October 25, several leading Wall Street bankers met to find a solution to the panic and chaos on the trading floor. The meeting included Thomas W. Lamont, acting head of Morgan Bank; Albert Wiggin, head of the Chase National Bank; and Charles E. Mitchell, president of the National City Bank. They chose Richard Whitney, vice president of the Exchange, to act on their behalf. With the bankers' financial resources behind him, Whitney placed a bid to purchase a large block of shares in U.S. Steel at a price well above the current market. As amazed traders watched, Whitney then placed similar bids on other "blue chip" stocks. This tactic was similar to a tactic that ended the Panic of 1907, and succeeded in halting the slide that day. In this case, however, the respite was only temporary.
Over the weekend, the events were covered by the newspapers across the United States. On Monday, October 28, more investors decided to get out of the market, and the slide continued with a record loss in the Dow for the day of 13%. The next day, "Black Tuesday", October 29, 1929, 16.4 million shares were traded, a number that broke the record set five days earlier and that was not exceeded until 1969. Author Richard M. Salsman wrote that on October 29—amid rumors that U.S. President Herbert Hoover would not veto the pending Hawley-Smoot Tariff bill—stock prices crashed even further." William C. Durant joined with members of the Rockefeller family and other financial giants to buy large quantities of stocks in order to demonstrate to the public their confidence in the market, but their efforts failed to stop the slide. The DJIA lost another 12% that day. The ticker did not stop running until about 7:45 that evening. The market lost $14 billion in value that day, bringing the loss for the week to $30 billion, ten times more than the annual budget of the federal government, far more than the U.S. had spent in all of World War I.>>