The motivation of the Catholic Church to adjusting the calendar was the celebration of Easter. The Council of Trent approved the plan to correct the errors in the calendar.First, it was necessary to approximate the correct length of a solar year.The second stage was to devise a model based on the approximation which would provide an accurate yet simple, rule-based calendar
Before the Gregorian calendar, most countries relied on the Julian calendar, which was introduced by Julius Caesar in 45 BC. It was in common use until the 1500s. The Julian calendar creates an error of 1 day every 128 years.
The Gregorian calendar was proposed by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII in accordance with the instructions from the Council of Trent (1545-1563) to correct for errors in the older Julian calendar. It was decreed by Pope Gregory XIII in a papal bull on February 24 1582. This bull was named “Inter Gravissimas” after its first two words.