Take a leap

This is a leap-year February. Why do we need to add an extra day sometimes?

 The name "leap year" probably comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, the day of the week in the 12 months following the leap day (from March 1 through February 28 of the following year) will advance two days due to the extra day (thus "leaping over" one of the days in the week

It takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the Sun. However, our Gregorian calendar has only 365 days in a year. If we didn't add a leap day on February 29 nearly every four years, we would lose almost six hours off our calendar every single year. After only 100 years, our calendar would be off by around 24 days.

source  wikipedia