The Gregorian Calendar, also known as the “Western Calendar” or “Christian Calendar”, is the most widely used calendar around the world today.
The Gregorian Calendar's predecessor, the Julian Calendar (*), was replaced because it did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year or solar year.
Too many Leap Years
The reason why the Julian Calendar was out of step with the tropical year was the rule it used to define a Leap Year. The Julian Calendar had only one rule to determine whether a year would have 29 days in February instead of 28. If the year could be divided by 4 then it was considered to be a Leap Year. The Gregorian Calendar on the other hand has a more complicated rule for calculating what years will be Leap Years:
- The year must be evenly divisible by 4;
- If the year can be evenly divided by 100, it is not a Leap Year, unless;
- The year is also evenly divisible by 400. Then it is a Leap Year.
These rules result in fewer leap years, minimizing the inaccuracies of the Julian Calendar.
http://www.timeanddate.com/calendar/julian-gregorian-switch.html