The Stress Management Center and Phobia Institute estimates between $700 million and $800 million are lost every Friday the 13th because of people's refusal to travel, purchase major items or conduct business. However, fear not, it’s only one day once in a blue moon. But more importantly, the S&P’s 500 has generally performed well over the last 20 years on this most ominous of days. The last Friday the 13th was in November 2009. On that day, the S&P gained 0.57 percent. In fact, the index has been higher on seven of the last eight Friday the 13ths. And the average change on those days has been up 0.34 percent, with positive returns 68 percent.