There's no evidence traders are superstitious about Friday the 13th, and there's no reason they should be, because returns on that day are perfectly normal. But one particular day of the month does actually stand out from the rest - in a positive way.

• The 16th of the month has the distinction of being the best performing day for stocks, going back decades.
• And the 1st of each month is a consistent outperformer, over time, as well
• Data suggests consistent outperformance on those 2 days, month after month.

The "turn of the month" effect …..  was first publicized in a 1988 paper, which found that the first four days of the month accounted for all the positive return in the Dow from 1897 to 1986. Another paper found a similar effect from 1926 to 2005. Our data suggest a similar effect from 1980 through 2017. The trends even applied for the period from 2008 to present. The cause of the effect isn't clear.

The middle of the month effect propping up the 16th …..  appears to be relatively new, and is often attributed to a "payday effect" because of the invention of 401(k) plans in the late 1970s, when millions of Americans began pouring their earnings into the stock market at the same time each month. Median daily returns from 1950 to 1979 did not show any bias to mid-month outperformance. However, since 1980, the 16th of the month has been the biggest buying day.

"The growth of 401(k) accounts and automatic deductions at payday is certainly a factor at play," said Paul Hickey, co-founder of Bespoke Investment Group. "And it has actually become more pronounced since the 1980s when 401(k) plans really started to become more mainstream."