“Friggatriskaidekaphobia.” You almost certainly know what it is, even if you’ve never heard the tongue-tying, nine-syllable official term for it. It is the fear of Friday the 13ths, one of the most widespread and enduring superstitions in the world. And whether or not you think it’s silly, the phobia does actually affect us, dragging down economic productivity at least once each year.

It's Friday 13th!