Is Friday the 13th really more common? I counted 400 years to find out
The superstition claims Friday the 13th is unlucky. It turns out it's also, very slightly, the most likely — the 13th lands on a Friday more often than on any other weekday. I counted every month in a full 400-year Gregorian cycle (plain Date UTC math) to see how big that edge really is. The edge, in one line: over 400 years the 13th falls on a Friday 688 times — more than any other weekday — but…
A 400-year cycle of the Gregorian calendar revealed an intriguing statistical oddity about Friday the 13th. Despite the popular superstition, the research showed the 13th day was only very slightly more likely to fall on a Friday than any other weekday. Over four centuries, the 13th landed on Friday 688 times, while Sunday came in second with 687 occurrences.
The other weekdays were evenly split between 685, 684, and 684. The statistical edge for Fridays amounted to only about two months in 4,800 months. Though the bias was tiny, it existed due to quirks in the leap year rules. Every year contains a Friday the 13th, never more than three. In total, 171 years experienced a single Friday the 13th, 170 had two, and a slim 59 had three.
The calendar's 400-year loop made this data exact rather than estimated. Leap years added an extra week in 71 of the 400 years, affecting payroll and planning cycles. The research was performed using a single Node script over the full 400-year period, yielding precise results without sampling.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
