"You can't have any [years] with none, and you can't have any with four, because of our funny calendar," mathemetician Underwood Dudley said.

In many ways the Gregorian calendar, which Pope Gregory XIII ordered the Catholic Church to adopt in 1582, works just like its predecessor, the Julian calendar—with a leap year every four years.

But the Gregorian calendar skips leap year on century years except those divisible by 400. For example, there was no leap year in 1900, but there was one in 2000. This trick keeps the calendar in tune with the seasons.

The result is an ordering of day-dates that repeats itself every 400 years, noted Dudley, a professor emeritus at DePauw University in Indiana, and author of Numerology: