Why was the Gregorian calendar invented?

The Gregorian calendar, widely adopted in the western world, was initially decreed by Pope Gregory XIII on 24 February 1582. The Gregorian calendar was first proposed by Aloysius Lilius because the mean year in the Julian Calendar was slightly long, causing the vernal equinox to slowly advance earlier in the calendar year.

On 5 October 1582, the Gregorian calendar was actively adopted in the western world for the first time. It required an adjustment to correct 11 accumulated days from the Julian calendar. The day following Thursday, 4 October 1582 was Friday, 15 October 1582, effective in most Catholic countries such as Italy, Poland, Spain and Portugal.