European scholars had been well aware of the calendar drift since the early medieval period. Bede, writing in the 8th century, showed that the accumulated error in his day was more than three days. Roger Bacon in c. 1200 estimated the error at seven or eight days. Dante, writing c. 1300, was aware of the need of a calendar reform. The first attempt to go forward with such a reform was undertaken by Pope Sixtus IV, who in 1475 invited Regiomontanus to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome. The increase of astronomical knowledge and the precision of observations towards the end of the 15th century made the question more pressing. Numerous publications over the following decades called for a calendar reform, among them a paper sent to the Vatican by the University of Salamanca in 1515, but the project was not taken up again until the 1540s, and implemented only under Pope Gregory XIII (r. 1572â1585).
The Gregorian calendar is internationally the most widely used civil calendar .    It is named after Pope Gregory XIII , who introduced it in October 1582.