Gregorian calendar.
The civil calendar now in use in most countries, introduced by Pope Gregory XIII in 1582 to replace the Julian calendar.
A workable civil calendar needs to be organized such that the seasons remain in step with the months of the year. This is a problem because the time taken by the Earth to orbit the Sun is not a whole number of days. The introduction of an extra day each fourth or leap year makes a first-order correction, but further adjustments are necessary if the calendar is to stay synchronized with the seasons over centuries.
In the Gregorian system, years exactly divisible by four are leap years, except that century years must be exactly divisible by 400 to be leap years. Thus 2000 is a leap year, but 1900 and 2100 are not. Averaged over 400 years, the rule gives an average year length of 365.2425, which is close to the true length of the tropical year, 365.2422 days.
The Gregorian calendar came into effect in Roman Catholic countries in October 1582, when the seasons were brought back into step by eliminating 10 days from the calendar. Thursday 4 October was followed by Friday 15 October. Also, on the introduction of the Gregorian system, the new year began on 1 January for the first time, instead of 25 March. Britain and its colonies did not introduce the Gregorian calendar until September 1752, by which time an 11-day correction was needed.