A leap year is a one which has got 366 days. In a leap year, February month contains 29 days. The leap year occurs once in four years. That is, every fourth year is a leap year. If the year number is divisible by 4, that particular year is known as leap year. To be more specific, the years 1984, 1988, 1996, and 2008 are all leap years, as all these numbers are divisible by 4. However, all century years (years ending with two zeroes), are not leap years, though the year numbers are divisible by 4. Only those century years whose year numbers are divisible by 400 are leap years. Hence, the years 1900, 2100, and 2200 are not leap years. But the years 2000, 2400, and 2800 are leap years. In order to understand as to why the concept of leap year is introduced, it is very much essential to know some details about tropical year, civil year and the evolution of calendar.