A team of analysts from Bespoke Investment Group crunched the numbers to show that the S&P 500 index tends to outperform on Fridays, and on the 13th day of the month. However, when the two occur together, on Friday the 13th, average gains for the S&P 500 were four times the historical average daily move dating back to 1953, when the modern five-day trading week was first adopted.
However, “Fridays have been much stronger with an average daily gain of 6.1 bps and positive returns 55.9% of the time. Even the 13th day of the month has been better than average with an average gain of 5.2 bps and positive returns 53.6% of the time,” the Bespoke team said. To be sure, Friday the 13th performance has historically varied from month to month. When it falls in October, average performance is notably weaker than other months.