When do we observe a leap year? Why?

Earth takes 365 days & 6 hours to revolve around the sun, which is one year. For the sake of convenience we take 365 days as one year and the 6 hours add up to 24 hours after 4 years, thus making it one additional day. This surplus day is added to the mouth of February. Thus every fourth such a year with 366 days is called a leap year.