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A calendar year begins on the
New Year's Day In the Gregorian calendar, New Year's Day is the first day of the calendar year, January 1, 1 January. Most solar calendars, such as the Gregorian and Julian calendars, begin the year regularly at or near the December solstice, northern winter ...
of the given
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single and specific day within such a system. A calendar is ...
system and ends on the day before the following New Year's Day, and thus consists of a whole number of days. The
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian cale ...
year, which is in use as civil calendar in most of the world, begins on
January 1 January 1 is the first day of the calendar year in the Gregorian Calendar; 364 days remain until the end of the year (365 in leap years). This day is also known as New Year's Day since the day marks the beginning of the year. __TOC__ Events ...
and ends on December 31. It has a length of 365 days in an ordinary year but, in order to reconcile the calendar year with the astronomical cycle, it has 366 days in a
leap year A leap year (also known as an intercalary year or bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) compared to a common year. The 366th day (or 13th month) is added to keep t ...
. With 97 leap years every 400 years, the Gregorian calendar year has an average length of 365.2425 days. Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the
Julian calendar The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts ...
has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Lunar Hijri calendar ("
Islamic calendar The Hijri calendar (), also known in English as the Islamic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the Ramad ...
") is a
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based on the solar year, and lunisolar calendars, whose lunar months are br ...
consisting of 12 lunar months in a year of 354 or 355 days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the calendar year in most calendars. A
year A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...
can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. This may be termed a "year's time", but is not a "calendar year".


Quarter year

The calendar year can be divided into four quarters, often abbreviated as Q1, Q2, Q3, and Q4. Since they are three months each, they are also called trimesters. In the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian cale ...
: * First quarter, Q1: January 1 – March 31 (90 days or 91 days in leap years) * Second quarter, Q2: April 1 – June 30 (91 days) * Third quarter, Q3: July 1 – September 30 (92 days) * Fourth quarter, Q4: October 1 – December 31 (92 days) In some domains, weeks are preferred over months for scheduling and reporting, so they use quarters of exactly 13 weeks each, often following ISO week date conventions. One in five to six years has a 53rd week which is usually appended to the last quarter. It is then 98 days instead of 91 days long, which complicates comparisons. In the
Chinese calendar The traditional Chinese calendar, dating back to the Han dynasty, is a lunisolar calendar that blends solar, lunar, and other cycles for social and agricultural purposes. While modern China primarily uses the Gregorian calendar for officia ...
, the quarters are traditionally associated with the 4
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt, tilted orbit around the Sun. In temperat ...
s of the year: * Spring: 1st to 3rd month *
Summer Summer or summertime is the hottest and brightest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, daylight hours are the longest and darkness hours are the shortest, with day ...
: 4th to 6th month *
Autumn Autumn, also known as fall (especially in US & Canada), is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September (Northern Hemisphere) or March ( Southern Hemisphe ...
: 7th to 9th month *
Winter Winter is the coldest and darkest season of the year in temperate and polar climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Dif ...
: 10th to 12th month


Quadrimester

The calendar year can also be divided into quadrimesters (from French ''quadrimestre''), lasting for four months each. They can also be called the early, middle, or late parts of the year. In the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian cale ...
: * First quadrimester, early year: January 1 – April 30 (120 days or 121 days in leap years) * Second quadrimester, mid-year: May 1 – August 31 (123 days) * Third quadrimester, late year: September 1 – December 31 (122 days)


Semester

The calendar year can also be divided into semesters, lasting six months each and often being abbreviated as S1 and S2. In the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian cale ...
: * First semester, S1: January 1 – June 30 (181 days or 182 days in leap years) * Second semester, S2: July 1 – December 31 (184 days)


See also

* * (historical usage) * * * * * * * *
Julian year (astronomy) In astronomy, a Julian year (symbol: a or aj) is a unit of measurement of time defined as exactly 365.25 days of SI seconds each.P. Kenneth Seidelmann, ed.''The explanatory supplement to the Astronomical Almanac'' (Mill Valley, Cal.: Universi ...
a time interval of exactly 365.25 Earth days * Julian year (calendar) a year in the
Julian calendar The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts ...
that is either 365 or 366 days, or 365.25 days on average * * * *


Notes


References

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Year A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...
Units of time Types of year