The Gregorian calendar is the most widely used civil calendar in the world. Adopted by Pope Gregory XIII in 1582 to correct the drift of the Julian calendar, it has 12 months, 365 or 366 days, and is used by 168 of the 195 countries we cover at caldays.
A short summary of what makes the Gregorian system tick — and why the rest of the world had to catch up.
The Julian calendar, introduced by Julius Caesar in 46 BC, assumed a year was exactly 365.25 days. It was close, but off by about 11 minutes per year — and over 16 centuries that drift accumulated to roughly 10 days.
By 1582 the spring equinox (which sets the date of Easter) had drifted from March 21 to March 11. Pope Gregory XIII issued the papal bull Inter Gravissimas, which (a) skipped 10 days — October 4 was immediately followed by October 15, 1582 — and (b) introduced the modern leap-year rule.
Different countries adopted the new calendar at different times. Catholic countries switched immediately. Protestant Britain and its colonies waited until 1752 (skipping 11 days). Russia held out until 1918, China until 1949.
A year is a leap year if it is divisible by 4 — unless it is divisible by 100 but not by 400. So 2000 was a leap year, 1900 was not, 2100 will not be, and 2400 will be. This rule averages to 365.2425 days per year, very close to the actual tropical year of 365.24219 days.
The next leap years are 2028, 2032, 2036, and 2040. 2026 is a common year — February has 28 days.