The earth takes approximately 365.242189 days to revolve around the sun so the existence of Leap year is justified. The earth moves around the sun once in about 365.25 days. Therefore, in essence, if we had a calendar consisting of exactly 365 days, our calendar would gradually slip away from synchronising with the seasons. We add one more day to the calendar every fourth year just so that it is adjusted according to the seasons. During this additional day, which is known as the leap day. In leap years, there are 366 days rather than 365 days.  

• This concept originated with the ancient Egyptians. The Egyptians were astronomer and farmers; they had to come up with a calendar that was predictable to help them know when exactly to plant or harvest their farm products.  
• It became evident throughout time that Egyptians observed a shift in the seasons which no longer followed their original calendar. They fixed this issue by adding one additional day to their calendar cycle every four years.  
•  the ancient Romans had a leap-year calendar. The Roman calendar wasn’t particularly accurate, and it gradually got out of step with the seasons. The highly-accurate modern-day calendar was adopted in 1582 by Pope Gregory XIII. The calendar that has become the world standard over the course of time is a modified variation of this same calendar referred to as the Gregorian calendar.

The Gregorian calendar uses the following rules to determine which years are leap years:

A leap year must occur every fourth year. Nevertheless; every fourth year except for the years which are divisible by one hundred is not a leap year. Except if that year is itself divisible by 400. In essence, the years 1600 and 2000 were leap years while the years 1700, 1800, and 1900 was not leap year.