Our recently arrived 2012 is a leap year, in which an extra day is added to the calendar at the end of February, giving it 29 days instead of the usual 28, in order to synchronize it with the seasons.
Leap years are necessary to keep calendars working properly. According to the website infoplease.com, the 365 days of the annual calendar are meant to match up with the solar year.
A solar year is the time it takes the earth to complete its orbit around the sun -- about one year.
But the actual time it takes for the earth to travel around the sun is in fact a little longer than that -- about 365 1/4 days (365 days, 5 hours, 48 minutes, and 46 seconds, to be precise). So the calendar year is a touch shorter than the solar year.
After four years the four extra quarter days would make the calendar fall behind the solar year by about a day. Over the course of a century, the difference between the solar year and the calendar year would become 25 days. Instead of summer beginning in June, for example, it wouldn't start until nearly a month later, in July.
But it gets more complicated. The exact length of a solar year is actually 11 minutes and 14 seconds less than 365 1/4 days. That means that if you add a leap day every four years, the calendar would overshoot the solar year by 11 minutes and 14 seconds per year. After 128 years, the calendar would gain an entire extra day.
To deal with this, the creators of the Gregorian calendar (introduced in 1582) decided to omit leap years three times every four hundred years. This would shorten the calendar every so often and rid it of the annual excess of 11 minutes and 14 seconds. So in addition to the rule that a leap year occurs every four years, another rule instructs that a century year is not a leap year unless it is evenly divisible by 400 -- which eliminates three leap years every few hundred years.
For those who are interested in the equations that determine this stuff, visit the website Science World by clicking here.