Leap Day is an additional day added in the month of February in order to balance the Gregorian calendar with an annual 6-hour lag in the Earth's rotation around the Sun. To make up for this lag time, an additional day is added to the month of February in years evenly divisible by the number 4.
Extra Day
It takes the Earth 365 days and 6 hours to complete a rotation around the Sun. If the Gregorian calendar were followed strictly, without the addition of a leap day, then the Earth's relation to the Sun would fail to coincide with established astrological schedules.Time and Date: Why Leap Years Are Used