Currently in use, the name of this calendar comes from Pope Gregory XIII, who reformed the Julian calendar in 1582. The Julian year was 365 days and 6 hours. By the sixteenth century the total surplus time had displaced the vernal equinox to 11 March 11 from 21 March, the date adopted in the fourth century. Pope Gregory XIII fixed the problem, by removing ten days in the year 1582 and proclaiming that thereafter an extra day would be added every four years (leap year). In 1582, the lag of the Julian calendar in relation to the sun was ten days. Hence, 4 October 1582 was followed by 15 October to correct the disparity. The reform was adopted almost immediately in most Roman Catholic countries but more gradually in Protestant countries. It wasn't adopted in England and her colonies in America until 3 September 1752. While the year there until 1752 began on 25 March, the changeover to 1 January as New Year's Day was also effected that year.
Gre·go·ri·an cal·en·dar / grəˈgôrēən/ • n. the calendar introduced in 1582 by Pope Gregory XIII, as a modification of the Julian calendar.