The Julian Calendar’s intercalation rule called for an extra day in February every four years without exception. This provided for almost eight more intercalary days than necessary in each thousand years. As a result, the calendar continued to became further out of phase with seasons as time passed by. As early as the thirteenth century of this era, Roger Bacon and others pointed out that some calendar change was needed, but nothing was done for many years to correct it.
By the mid sixteenth century, actual moon phases were occurring four days prior to those in ecclesiastical tables, so Easter was sometimes being celebrated prior to the vernal equinox rather than following it as had been specified by the A.D. 325 Council of Nicaea. A decree on fasts and feast days that addressed this problem was approved in A.D. 1563 at the Council of Trent.
According to the Trent council, two main things needed to be accomplished regarding the calendar: reestablish the vernal equinox date to not occur much later than March 21 and determine the Easter moon’s 14th day. Easter would then be observed on the following Sunday. Following some of the council’s recommendations, Pope Pius V modified both the intercalation rule and lunar tables used to determine Easter’s date.
Pius V’s successor was Pope Gregory XIII, who had been a representative of Paul III at the Trent Council. Aloysius Lilius and others urged Gregory to complete the Trent recommendations. In Gregory’s tenth year this step was taken. Lilius, considered to be principal author of the Gregorian Calendar, had proposed that the seasonal error should be corrected by dropping ten intercalations during the following forty years. This was apparently based on his observation that vernal equinoxes then occurred on March 11.
Others, including mathematician and astronomer Christopher Clavius (shown on the left) agreed with the ten-day correction but believed that they should be dropped all at once. Gregory took this advice and shortened October of A.D. 1582 by ten days. Thursday, October 4, 1582 (Julian) was followed immediately by Friday, October 15, 1582 (Gregorian).
Gregory completed modification of the old intercalation rule by specifying that any year whose number ended with 00 must also be evenly divisible by 400 in order to have a 29-day February. He also changed the ecclesiastical table of moon phases used to help determine Easter’s date.
Regardless of these improvements to the formula and table values used, and because lunations are out of sync with Earth’s revolution around the sun, Easter is observed in some years during March and others in April.
Except for predominently Catholic countries, nations were slow in adopting Gregory’s changes. For a list of nations and the dates they adopted the Gregorian Calendar see this page. Among the last European power to accept it was Great Britain. That empire and its colonies (including those in America) put it into effect in 1752. By then, in order to stay in sync with nations that had already adopted Gregory’s changes, the switch involved eliminating eleven days from the old Julian Calendar. At the same time, they changed the beginning of their legal year from March 25 to January 1.
Because of Gregory’s refinement of the leap year rule, our Western calendar has almost kept pace with seasons.
As time goes by, correspondence between seasons and Gregorian Calendar dates vary, and tend to become further removed. This is because Gregory’s leap year rule is a rigid formula whose calculated results do not match year lengths over extended periods of time. One way to keep the calendar more closely coordinated with seasons over the long term is to use a method that is based on observation. This can be done with A Generalized Leap Year Rule.