For those who want to know all about the history of our calendar...

Calendars based on the movements of the sun and moon have been in practice since ancient times, however until recently, none have been truly accurate.

One of the most accurate calendars to date was created by the Romans. The Julian calendar, as it was called, was authorized by Julius Caesar in 46 B.C. in the year 709 of
Rome. The Julian calendar was based on the assumption that the length of a year was exactly 365.25 days long. Thus, in order to simplify things, each year was 365 days, and
every fourth year, called a leap year, was given an extra day to make up for the lost .25 days on the standard years.

The Venerable Bede, an Anglo-Saxon monk, announced in A.D. 730 that the Julian year was 11 minutes and 14 seconds too long. This meant that an extra day was being added every
128 years. Although Bede's finding's were proven accurate, nothing was done about the miscalculation for more than 800 years. By 1582, the accumulated error was estimated to
amount to 10 days.

In that year Pope Gregory XIII decreed that the day following October 4, 1582, should be called October 15, thus dropping 10 days from the Julian calendar, and initiating our
current calendar, which is aptly called the Gregorian calendar. (Personally, I think the credit should be given to Bede, but who's to argue with the pope?)

Even with this new calendar the error in the length of the year would have recurred at the rate of a little more than 3 days every 400 years. Therefore, 3 of every 4
centesimal years (years ending in 00) were made common years (365 days), not leap years. Thus, 1600 was a leap year but 1700, 1800, and 1900 were not. The year 2000 will be a
leap year, where 2100, 2200, and 2300 will not be. Leap years are those years divisible by 4, except centesimal years, which are common unless divisible by 400.