I downloaded SPX data from Yahoo to check for major gaps from the previous close to the next open. The 50 biggest ones, both up and down, occurred in the 1950s and there were only a few more than 2%. In those days a 5% move could be about a point or two. I'm sure anything is possible, but what are the real chances of a major percentage gap these days? Edit: Even after the earthquake in Japan two weeks ago the S&P only opened 3 points lower on Monday than it closed the Friday of the quake (IIRC the quake happened just about closing time).