Looking at some of the specialized vocabulary of leap year.
If only Mother Nature had been just a little neater on this one and gotten the day and the year to line up together a little better. Then we wouldn't have all this leap year stuff to deal with.
But no, the earth's trip around the sun, which determines the year, cannot be measured in a neat whole number of rotations of the earth on its axis, which determines the day.
The solar year is approximately 365-1/4 days long, and our current solution to this odd little fraction is to add an extra day every four years – except for round century years (the double-zero ones like 1900) unless they are divisible by 400. Thus the year 2000 was not only "Y2K," with all that meant to strike fear into the hearts of IT departments everywhere; it was a leap year, to boot.
And so is 2008. Today, Friday, Feb. 29, is leap year day – an intercalary day, inserted to keep the calendar, and us, on track.
The idea is that a given date needs to mean something on the calendar from year to year. The vernal equinox, when day and night are equal, happens about March 21 in the Northern Hemisphere. So the relationship between the little numbered boxes in our daybooks and the movements of the heavenly bodies is more or less constant.
In a calendar without a provision for leap year, as in the Muslim lunar calendar, a given date "migrates" across the year. Thus the fasting month of Ramadan sometimes occurs during the high summer, when abstaining from water during the heat of the day represents a particular sacrifice and even a danger.
In a nomadic society, the lack of fixity wasn't much of a problem, but agrarian societies found it useful to plant and harvest according to a fixed calendar.