Gregorian calendar

GregorianN.S.NSChristian calendarcalendarGregorian dateGregorian yearWestern calendarnew calendarN.S
The Gregorian calendar is the calendar used in most of the world.wikipedia
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Anno Domini

ADBCChristian era
There was no discontinuity in the cycle of weekdays or of the Anno Domini calendar era.
The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars.

Calendar era

erayear numberingepoch
There was no discontinuity in the cycle of weekdays or of the Anno Domini calendar era.
For example, the Gregorian calendar numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras).

Leap year

leap yearsbissextile29 February
The calendar spaces leap years to make the average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. A regular Gregorian year consists of 365 days, but in certain years known as leap years, a leap day is added to February.
For example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28.

Dual dating

1440/4116 February/1 March 19171727/8
To unambiguously specify a date during the transition period, (or in history texts), dual dating is sometimes used to specify both Old Style and New Style dates (abbreviated as O.S and N.S. respectively).
For example, in "10/21 February 1750/51", the dual day of the month is due to the correction for excess leap years in the Julian calendar by the Gregorian calendar, and the dual year is due to some countries beginning their numbered year on 1 January while others were still using another date.

Old Style and New Style dates

O.S.Old StyleNew Style
To unambiguously specify a date during the transition period, (or in history texts), dual dating is sometimes used to specify both Old Style and New Style dates (abbreviated as O.S and N.S. respectively).
There were two calendar changes in Great Britain and its colonies, which may sometimes complicate matters: the first was to change the start of the year from Lady Day (25 March) to 1 January; the second was to discard the Julian calendar in favour of the Gregorian calendar.

January

Mutsuki
January is the first month of the year in the Julian and Gregorian calendars and the first of seven months to have a length of 31 days.

February

modern month
February is the second (and shortest) month of the year in the Julian and Gregorian calendar with 28 days in common years and 29 days in leap years, with the quadrennial 29th day being called the leap day.

Common Era

CEBCEC.E.
The calendar era carries the alternative secular name of "Common Era".
Both notations refer to the Gregorian calendar (and its predecessor, the Julian calendar).

Solar calendar

solarsun calendarastronomical calendar
The Gregorian calendar is a solar calendar with 12 months of 28–31 days each.
The Gregorian calendar, widely accepted as standard in the world, is an example of a solar calendar.

July

July is the seventh month of the year (between June and August) in the Julian and Gregorian Calendars and the fourth of seven months to have a length of 31 days.

September

Septembre
September is the ninth month of the year in the Julian and Gregorian calendars, the third of four months to have a length of 30 days, and the fourth of five months to have a length of less than 31 days.

October

Roctober
October is the tenth month of the year in the Julian and Gregorian Calendars and the sixth of seven months to have a length of 31 days.

November

November is the eleventh (and penultimate) month of the year in the Julian and Gregorian Calendars, the fourth and last of four months to have a length of 30 days and the fifth and last of five months to have a length of fewer than 31 days.

December

No Gender December
December is the twelfth and final month of the year in the Julian and Gregorian Calendars and is the seventh and last of seven months to have a length of 31 days.

Calendar

calendarscalendricalcalendar system
The Gregorian calendar is the calendar used in most of the world.
The first calendar reform of the early modern era was the Gregorian calendar, introduced in 1582 based on the observation of a long-term shift between the Julian calendar and the solar year.

February 29

leap day29 FebruaryLeap Year Day
A regular Gregorian year consists of 365 days, but in certain years known as leap years, a leap day is added to February.
A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in most of the world.

Computus

date of Eastercalculating EasterEaster table
The reform also altered the lunar cycle used by the Church to calculate the date for Easter (computus), restoring it to the time of the year as originally celebrated by the early Church.
Because the date is based on an ecclesiastical or paschal "equinox" date rather than the actual astronomical equinox, there are differences between calculations done according to the Julian calendar and the modern Gregorian calendar.

Equinox

autumnal equinoxautumn equinoxequinoxes
The calendar was developed as a correction to the Julian calendar, shortening the average year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.
This drift induced Pope Gregory XIII to create the modern Gregorian calendar.

Inter gravissimas

It was instituted by papal bull Inter gravissimas dated 24 February 1582 by Pope Gregory XIII, after whom the calendar is named.
The reform came to be regarded as a new calendar in its own right and came to be called the Gregorian calendar, which is used in most countries today.

Easter

Easter SundayPaschaEaster Day
Although a recommendation of the First Council of Nicaea in 325 specified that all Christians should celebrate Easter on the same day, it took almost five centuries before virtually all Christians achieved that objective by adopting the rules of the Church of Alexandria (see Easter for the issues which arose).
Easter and the holidays that are related to it are moveable feasts which do not fall on a fixed date in the Gregorian or Julian calendars which follow only the cycle of the Sun; rather, its date is offset from the date of Passover and is therefore calculated based on a lunisolar calendar similar to the Hebrew calendar.

March equinox

vernal equinoxspring equinoxNorthward equinox
In 1545, the Council of Trent authorized Pope Paul III to reform the calendar, requiring that the date of the vernal equinox be restored to that which it held at the time of the First Council of Nicaea in 325 and that an alteration to the calendar be designed to prevent future drift.
On the Gregorian calendar, the Northward equinox can occur as early as 19 March or as late as 21 March at Greenwich.

Christopher Clavius

ClaviusChristoph ClaviusChristof Klau
The German mathematician Christopher Clavius, the architect of the Gregorian calendar, noted that the tables agreed neither on the time when the Sun passed through the vernal equinox nor on the length of the mean tropical year.
Christopher Clavius (25 March 1538 – 6 February 1612 ) was a Jesuit German mathematician and astronomer who modified the proposal of the modern Gregorian calendar after the death of its primary author, Aloysius Lilius.

Aloysius Lilius

Luigi LilioAloisius LiliusLuigi Giglio
The reform adopted was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio).
1510 – 1576), also variously referred to as Luigi Lilio, Luigi Giglio, was an Italian doctor, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that (after modifications) became the basis of the Gregorian Calendar reform of 1582.

Islamic calendar

AHHijriHijri calendar
Prior to 1917, Turkey used the lunar Islamic calendar with the Hegira era for general purposes and the Julian calendar for fiscal purposes.
The civil calendar of almost all countries where the religion is predominantly Muslim is the Gregorian calendar, with Syriac month-names used in the Levant and Mesopotamia (Iraq, Syria, Jordan, Lebanon and Palestine).

New Year

New YearsNew Year's DayNew Year Festival
The Julian calendar, which began in 45 BC, continued to use 1 January as the first day of the new year.
In the Gregorian calendar, the most widely used calendar system today, New Year occurs on January 1 (New Year's Day).