When did leap year originate?
The Gregorian calendar is closely based on the Julian calendar, which was introduced by Julius Caesar in 45 BC. The Julian calendar featured a 12-month, 365-day year, with an intercalary day inserted every fourth year at the end of February to make an average year of 365.25 days. But because the length of the solar year is actually 365.242216 days, the Julian year was too long by .0078 days (11 minutes 14 seconds).
This may not seem like a lot, but over the course of centuries it added up, until in the 16th century, the vernal equinox was falling around March 11 instead of March 21. In 1582, Pope Gregory XIII adjusted the calendar by moving the date ahead by 11 days and by instituting the exception to the rule for leap years. This new rule, whereby a century year is a leap year only if divisible by 400, is the sole feature that distinguishes the Gregorian calendar from the Julian calendar.
Following the Gregorian reform, the average length of the year was 365.2425 days, an even closer approximation to the solar year. At this rate, it will take more than 3,000 years for the Gregorian calendar to gain one extra day in error. ~by Ann Marie Imbornoni