WHY DO WE HAVE LEAP YEARS?
We have a leap year because a standard year is not actually exactly 365 days long - it's 365.2422 days long, so slightly longer than we count in a standard year calendar.
That long number - 365.2422 - is the number of days it takes the planet Earth, on average, to make a full rotation around the sun.
So, an extra day is added onto our shortest month, about once every four years, in order to keep the calendar months in line with their assigned seasons. If we didn't add the day to compensate, the months would get so out of step with the seasons that eventually we'd be celebrating Christmas in the middle of summer.