What is the Gregorian Calendar?

   By 1582, under the Julian calendar, the beginning of spring had moved back to March 11, and it was once again necessary to adjust the date so it would line up with the seasons. Pope Gregory XIII introduced another change in the calendar in an attempt to narrow the gap between the Julian calendar of 365.25 days per year and the natural year consisting of 365.242199 days per year. First, Gregory declared that the current date would be set ahead ten days in order to bring the start of spring back to March 21. Then he proceeded to eliminate three days from every four centuries in the future. This was accomplished by modifying the leap-year rule, so that only one of every four century-years would be a leap year. Under the Julian calendar, every century-year (200, 300, 400, etc.) divisible by four was designated as a leap year. Gregory ruled that only those century-years divisible by four hundred (400, 800, 1200, etc.) would be a leap year. The new calendar was named the Gregorian calendar and is still in use today in most of the Western world.