Introduced by Pope Gregory XIII. The first day of the new calendar was Friday October 15, 1582.
The Gregorian Calendar established new rules for leap years to improve the accuracy over the old Julian calendar which had leap years every 4 years. Under the new rules, leap years now occur when the year is divisible by 4 and not 100, or the year is divisible by 400. Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not. Under this system the calendar should be accurate within a day for about 3322 years.
To use the calendar below, select a month from the drop down menu and type in the year (xxxx) in the field to the right, and then press the "Update" button.