The Gregorian calendar is a solar calendar and the most widely used calendar in the world. It was created towards the end of the 16th century by a reform of the Julian calendar. It is named after Pope Gregory XIII, who decreed it 1582 with a papal bull.
According to this calendar, the average length of the year is 365.2425 days; thus it is more precise than the 365.25 days of the Julian calendar.
Its years regularly comprise 365 days, which in turn are divided into 12 months of varying lengths. Most months have 30 or 31 days, with one month (February) having only 28 days. Every four years, a leap year is inserted in which February is 29 days long – so the leap year has 366 days.
The year 2100 is a special case according to the Gregorian calendar; it is exceptionally not a leap year.