The terms "leap year" and "leap day" have their origin in the calendar–or, rather, two versions of it.

The Gregorian Calendar, implemented by Pope Gregory XIII in 1582, is not the exact same length as a solar year, one Earth orbit around the Sun. That exact figure is 365 days, 5 hours, 48 minutes, and 46 seconds.

What this means is that a calendar year should be 365.25 days, or thereabouts. But how do you show a quarter of a day on a calendar? It is far easier to wait until four quarter-days have accumulated, making one full day, and then show that, which is what we now do and why every four years is a "leap year," which has one full extra day in it, February 29.

A rule of this "leap" function is that a century year, cannot be a leap year unless it is divisible by 400. So the latest example of this, the year 2000, was a leap year because 400 goes into 2000 evenly. The next century year that would otherwise be a leap year, 2100, will not be a leap year because 2100 is not divisible by 400. The same was true of 1700, 1800, and 1900. (By the same mathematics, 1600 was a leap year.

This rule is one of the distinctions between the Gregorian Calendar and the calendar it replaced, the Julian Calendar, created by Julius Caesar. The Julian Calendar had the idea of a leap day and a leap year, although February 23 was the day that was repeated every four years when people were using the Julian Calendar.

As far as the word "leap," it is what dates actually do in a leap year. If New Year's Day, January 1, is a Tuesday in a non-leap year, then January 1 is a Wednesday in the following year, provided that that year is also a non-leap year. However, in a leap year, the day of the week advances two days; so if January 1 is on a Tuesday in a non-leap year and then the following year is a leap year, then January 1 in that year would be on a Thursday–in effect, "leaping" over Wednesday in the process.

Have a suggestion for this feature? Email Dave.