Stock traders should fear Friday the 13th - especially in October

If you were thinking about buying stocks today - on the dreaded Friday the 13th - you may want to think again.

Since 1928, there have been 152 Friday the 13ths, on which the S&P 500 returned an annualized 4.2% gain. That's only roughly one-third the increase for all Fridays over time, according to data compiled by Ryan Detrick of LPL Financial.

That's fine, right? After all, a gain is a gain?

Not quite.

When Detrick breaks down the analysis month, the picture gets much starker for Friday the 13ths falling in the month of October. Across the 12 times it's happened - including the most recent one, in 2006 - the S&P 500 has fallen by an average of 0.54%. That's the second-worst out of any month, trailing only November, Detrick finds.

So should you just stay out of the market entirely as you wait for Friday the 13th to pass? It probably doesn't matter one way or another, says Detrick. After all, in the end, it's just a minor market quirk with a very limited sample size.

"Unless you break a mirror or see a black cat on Friday, we aren't in any way saying one day matters more or less than another," Detrick wrote in a blog post. "Still, wouldn't you know it- Friday the 13th tends to be a weak day on average; but taking it a step further, this day does even worse during October. You can't make this stuff up!"