A YEAR is typically defined as 365 days, or roughly the length of time that it takes the Earth to orbit around the Sun once but in reality the Earth doesn’t complete it’s orbit around the sun in precisely 365 days, its actually closer to 365.2524 days (“a vernal equinox year”).
To make up for the ¼ of a day difference, a corrective measure or an extra day known as February 29, is usually added to the calendar every four years, and that day is called a leap day.
In 46 BC, Roman emperor Julius Caesar introduced the Julian calendar, which was an amended version of the Roman calendar, with an extra leap day (or intercalary day) every fourth year.
The Julian calendar would’ve worked out perfectly except the Earth’s obit around the sun takes 365.2425 days and not precisely 365.25 days. So over a century, this small difference accumulated to a slightly more than three-quarters of a day. From the time the Julian calendar was introduced to the 16th Century AD, the beginning of spring shifted from March 23 to March 11.
In 1582 Pope Gregory XIII established the Gregorian calendar and introduced the century rule to correct the rounding error. If a leap year falls at the beginning of a century, a year ending in double zeroes, you only add a leap day if it’s divisible by 400. The century years 1600, 2000 and 2400 are leap years, but years 1700, 1800, and 1900 are not. The Gregorian calendar also shifted the beginning of spring to March 21.