What Weekday Is Your Birthday, Really? The Calendar Is Quietly Biased

You'd expect a date to fall on each of the seven weekdays about equally often. It doesn't — and it can't. The Gregorian calendar runs in an exact 400-year cycle, and 400 isn't divisible by 7, so every fixed date has weekdays it prefers and weekdays it avoids. Because the cycle is exact, these aren't estimates from a big sample — they're the...