A leap year consists of 366 days, as opposed to a common year, which has 365 days. Nearly every 4 years is a Leap Year, and we add a Leap Day, an extra day on February 29. Hence, the question arises why do we need to add one day every four year to make it a leap year? Let's find out .......

The calendar year is synchronized with the astronomical or seasonal year. The astronomical year or seasonal year is the time taken by the earth to go exactly once around the sun - that is 365 days. But in reality it takes little more time than 365 days. Precisely, it takes the earth approximately 365.242199 days - or 365 days, 5 hours, 48 minutes, and 46 seconds to circle around the sun. This 5hrs and few minutes difference adds up to make a difference of full one day every 4 years. That is why every four years gregarion calender adds one more day in february.

This might seems like a little fractional (almost 6 hours or 1/4 of a day) difference that shouldn't matter much within a short span of time. But as the time span increases,say,200 years, the calender year lags behind 200 days or almost 6 and 1/2 months to the astronomical year, which is huge. This means that we could have a whole seasonal shift (since the seasons are tied to the astronomical year, because they depend on the earth's slant relative to the sun), Christmas would come in the autumn . . . and then in summer . . . and . . .

To prevent this drift between the calendar year and the astronomical (seasonal) year, we add one extra day every four years. It was implemented by Pope Gregory in March 1582. Pope Gregory invented the "Gregorian Calendar", which is followed as the standard calender across the world.