intercalation

Information about intercalation

Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require a combination of both adjustments.

The solar year does not have whole number of days, but a calendar year must have a whole number of days. The only way to reconcile the two is to vary the number of days in the calendar year.

In solar calendars, this is often done by adding to a common year of 365 days, an extra day (leap day or intercalary day): this makes a leap year of 366 days.

The Decree of Canopus, which was issued by the pharaoh Ptolemy III, Euergetes of Egypt in 239 BC, decreed a solar leap day system.

In the Julian Calendar as well as in the Gregorian Calendar that improved it, intercalation is done by adding an extra day to February in each leap year. In the Julian Calendar this was done every 4 years. In the Gregorian calendar years whose number is evenly divisible by 100 but not 400, were exempted in order to improve accuracy.

The solar year does not have a whole number of lunar months either, so a lunisolar calendar must have a variable number of months in a year. This is usually 12 months, but sometimes a 13th month (an intercalary or embolismic month) is added to the year.

ISO 8601 includes a specification for a 52-week year. Any year that has 53 Thursdays has 53 weeks; this extra week may be regarded as intercalary.

The determination of whether a year has intercalation may be calculated (Julian, Gregorian and Hebrew calendars), or determined by observation (Iranian calendar).

See also

lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the
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A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit.
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According to the Gregorian calendar, the calendar year begins on January 1 and ends on December 31. Other alignments of the 12-month period can be used for purposes of accounting (see fiscal year).
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For the fictional reckoning of the World of Greyhawk, see .
A common year is a year that is a common calendar year. It has exactly 365 days and so is not a leap year. More generally it is a calendar year without intercalation.
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For the 1921 film starring Fatty Arbuckle, see .
A leap year (or intercalary year) is a year containing one or more extra days (or, in case of lunisolar calendars, an extra month) in order to keep the calendar year synchronised with the
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The Decree of Canopus (Stone of Canopus) is the memorial stone stele inscribed by the Pharaoh Ptolemy III Euergetes in 239 BC.

This is the earliest of the series of bilingual inscriptions of the "Rosetta Stone Series", the next being the Decree of Memphis (Ptolemy IV), for
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Ptolemy III Euergetes, (Greek: Πτολεμαίος Ευεργέτης, reigned 246 BC–222 BC) is sometimes called Ptolemy III Euergetes I.
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Gumhūriyyat Miṣr al-ʿArabiyyah
Arab Republic of Egypt


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Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the
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Gregorian calendar is the most widely used calendar in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull
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For the 1921 film starring Fatty Arbuckle, see .
A leap year (or intercalary year) is a year containing one or more extra days (or, in case of lunisolar calendars, an extra month) in order to keep the calendar year synchronised with the
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A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit.
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In lunar calendars, a lunar month is the time between two successive similar syzygies (new moons or full moons). There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction
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lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the
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For the Egyptian hawk-god, see .


The month is a unit of time, used with calendars, which is approximately as extensive as some natural period related to the motion of the Moon.
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--TZ
Example separate date and time in UTC:
-- Z
Example date only:
--
Example date with week number:

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Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the
..... Click the link for more information.
Gregorian calendar is the most widely used calendar in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull
..... Click the link for more information.
Hebrew calendar (Hebrew: הלוח העברי‎) or Jewish calendar is the calendar used by Jews for religious purposes.
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Iranian calendar (Persian: سالنمای هجری خورشیدی) also known as Persian calendar or the Jalāli Calendar
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calendar is a system for naming periods of time, typically days. These names are known as calendar dates. Cycles in a calendar are often synchronised with the perceived motion of astronomical objects.

A calendar is also a physical device (often paper).
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Bahá'í calendar, also called the Badí‘ calendar, used by the Bahá'í Faith, is a solar calendar with regular years of 365 days, and leap years of 366 days. Years are composed of 19 months of 19 days each, (361 days) plus an extra period of "Intercalary Days" (4 in
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Ayyám-i-Há refers to a period of four or five intercalary days in the Bahá'í calendar, where Bahá'ís celebrate the Festival of Ayyám-i-Há.[1] The four or five days occur in between the 18th and 19th months of the calendar from February 26 to March 1 and allow for
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Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the
..... Click the link for more information.
Gregorian calendar is the most widely used calendar in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull
..... Click the link for more information.
Iranian calendar (Persian: سالنمای هجری خورشیدی) also known as Persian calendar or the Jalāli Calendar
..... Click the link for more information.
Hebrew calendar (Hebrew: הלוח העברי‎) or Jewish calendar is the calendar used by Jews for religious purposes.
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Hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar. In Pakistan it is called 'desi' or native calendar.
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Chinese calendar is a lunisolar calendar, incorporating elements of a lunar calendar with those of a solar calendar. In China today, the Gregorian calendar is used for most day to day activities, but the Chinese calendar is still used for marking traditional Chinese holidays such
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