This is an entertaining way to explain it.
http://m.youtube.com/watch?gl=CA&hl=en&client=mv-google&v=56zlm9qhVGc
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