Gregorian calendar

The Gregorian calendar is a solar calendar introduced by Pope Gregory XIII in 1582. Its leap-year rules keep calendar dates aligned with the seasons more closely than the Julian calendar.

Leap year: A calendar year containing an extra day or, in some calendars, an extra month. The Gregorian calendar adds February 29 in years divisible by four, with century-year exceptions.
Pope Gregory XIII: The pope from 1572 to 1585 who promulgated the Gregorian calendar reform. His 1582 papal bull established the calendar and its initial date changes.
Civil calendar: A calendar adopted by a government or society for official dates and everyday use. The Gregorian calendar is the international standard for civil dates.
Gregorian calendar adoption: The gradual replacement of the Julian calendar by the Gregorian calendar in different countries and institutions. Adoption occurred across centuries rather than on one worldwide transition date.
Revised Julian calendar: A calendar used by some Orthodox churches, with leap-year rules that differ from the Gregorian system. It offers a distinct reform of the Julian calendar and currently matches Gregorian dates for many centuries.
Solar year: The time Earth takes to complete one cycle of seasons, approximately 365.2422 days. The calendar’s average year approximates this seasonal cycle.
Inter gravissimas: The 1582 papal bull that promulgated the Gregorian calendar reform. It set out the reform’s leap-year rules and the transition from the Julian calendar.
Computus: The calculation of the date of Easter in Christian calendars. Gregorian computus combines the calendar’s rules with lunar tables to determine Easter.
British calendar change of 1752: The 1752 British Empire transition to the Gregorian calendar, which omitted eleven dates from September. It illustrates how calendar reform could alter official dates within a country.
Islamic calendar: A lunar calendar of twelve months, used to determine Islamic religious dates. Its months follow lunar phases rather than the Gregorian calendar’s seasonal year.

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