What is the main reason for leap year?

TL;DR; The simple reason for leap year is that the earth revolves around the sun in approximately 365. read more

The simple reason for leap year is that the earth revolves around the sun in approximately 365.25 days, and so to make sure that a calendar year is the same as a tropical year (which is the period of earth's revolution), there needs to be an extra day added every 4 years. read more

Adding one day per four years didn't perfectly correct the problem, however, because Earth orbits in 11 minutes and 14 seconds less than 365 1/4 days. A correction, established in 1582, adds a leap day only to century years divisible by 400. Thus 2000 was a leap year, but 1900 wasn't a leap year. read more

February 29 is a date that usually occurs every four years, and is called leap day. This day is added to the calendar in leap years as a corrective measure, because the Earth does not orbit the sun in precisely 365 days. The Gregorian calendar is a modification of the Julian calendar first used by the Romans. read more