A calendar is a system of organizing Organizing is the act of rearranging elements following one or more rules days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time Time has been defined as the continuum in which events occur in succession from the past to the present and on to the future. Time has also been defined as a one-dimensional quantity used to sequence events, to quantify the durations of events and the intervals between them, and to quantify and measure the motions of objects and other changes, typically days A day is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second, weeks A week is a time unit equal to seven days, originating in the calendar of the Jews in the 6th century BC and adopted in the Roman, Muslim and various other calendars, months A month is a unit of time, used with calendars, which was first used and invented in Egypt, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have, and years A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic. The name given to each day is known as a date A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 May 2010" is ten days after "14 May 2010" in the Gregorian calendar. The date of a particular event. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is or the moon The Moon is Earth's only natural satellite[nb 4] and is the fifth largest satellite in the Solar System. It is the largest natural satellite in the Solar System relative to the size of its planet, a quarter the diameter of Earth and 1/81 its mass, and is the second densest satellite after Io. It is in synchronous rotation with Earth, always. Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs.

A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerized systems, which can be set to remind the user of upcoming events and appointments.

A calendar can also mean a list planned events, such as a court calendar A docket is the official schedule of hearings in a court of law. The term originally referred to the large folio books in which clerks recorded all filings and court proceedings for each case. Rules of civil procedure often state that the court clerk shall record specific bits of information "on the docket" when a specific event occurs.

The English word calendar is derived from the Latin word kalendae, which was the Latin name of the first day of every month.^[1]

Calendar systems

A full calendar system has a different calendar date for every day. Thus the week A week is a time unit equal to seven days, originating in the calendar of the Jews in the 6th century BC and adopted in the Roman, Muslim and various other calendars cycle is by itself not a full calendar system; neither is a system to name the days within a year A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic without a system for identifying the years.

The simplest calendar system just counts time periods from a reference date. This applies for the Julian day Julian day is used in the Julian date system of time measurement for scientific use by the astronomy community. Julian date is recommended for astronomical use by the International Astronomical Union. Virtually the only possible variation is using a different reference date, in particular one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.

Other calendars have one (or multiple) larger units of time.

Calendars that contain one level of cycles:

• week and weekday – this system (without year, the week number keeps on increasing) is not very common
• year and ordinal date within the year, e.g. the ISO 8601 ordinal date system ISO 8601 is an international standard covering the exchange of date and time-related data. The standard is titled "Data elements and interchange formats — Information interchange — Representation of dates and times" and was issued by the International Organization for Standardization . The purpose of this International Standard is to

Calendars with two levels of cycles:

• year, month A month is a unit of time, used with calendars, which was first used and invented in Egypt, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have, and day – most systems, including the Gregorian calendar The Gregorian calendar is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter gravissimas. The reformed calendar was adopted later that year by a handful of countries, with other countries (and its very similar predecessor, the Julian calendar The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus. It has a regular year of 365 days divided into 12), the Islamic calendar The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar based on 12 lunar months in a year of 354 or 355 days. It is the official calendar in Saudi Arabia and it's used to date events in many Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to, and the Hebrew calendar The Hebrew calendar or Jewish calendar is a lunisolar calendar used by Jews. Today, the calendar is used predominantly for Jewish religious observances. It is used to determine the dates for Jewish holidays, and also to determine appropriate public reading of Torah portions, yahrzeits (dates to commemorate the death of a relative), and daily Psalm
• year, week, and weekday – e.g. the ISO week date The ISO week date system is a leap week calendar system that is part of the ISO 8601 date and time standard. The system is used in government and business for fiscal years, as well as in timekeeping

Cycles can be synchronized with periodic phenomena:

• A lunar calendar is synchronized to the motion of the Moon The Moon is Earth's only natural satellite[nb 4] and is the fifth largest satellite in the Solar System. It is the largest natural satellite in the Solar System relative to the size of its planet, a quarter the diameter of Earth and 1/81 its mass, and is the second densest satellite after Io. It is in synchronous rotation with Earth, always (lunar phases A lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun. One half of the lunar surface is always illuminated by the Sun , and); an example is the Islamic calendar The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar based on 12 lunar months in a year of 354 or 355 days. It is the official calendar in Saudi Arabia and it's used to date events in many Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to.
• A solar calendar is based on perceived seasonal The need for farmers to predict seasonal events led to the development of calendars. However, the variability from year to year of seasonal events makes the seasonal year very hard to measure. This means that calendars are based on astronomical years (which are regular enough to be easily measured) as surrogates for the seasonal year. For example, changes synchronized to the apparent motion of the Sun; an example is the Persian calendar The Iranian calendars or Persian calendars can refer to any in a succession of a set of calendars invented or used for over two millennia in Greater Iran. One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes.
• A "luni-solar calendar" is based on a combination of both solar and lunar reckonings; an example is the traditional calendar of China and the Hindu Calendar The 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 India.
• There are some calendars that appear to be synchronized to the motion of Venus Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows. Because Venus is an inferior planet from Earth, it never, such as some of the ancient Egyptian Ancient Egypt was an ancient civilization of eastern North Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. The civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh, and it developed over the next three millennia. Its history calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass. The capitalized term Equator refers to the Earth's equator.
• The week A week is a time unit equal to seven days, originating in the calendar of the Jews in the 6th century BC and adopted in the Roman, Muslim and various other calendars cycle is an example of one that is not synchronized to any external phenomenon (although it may have been derived from lunar phases, beginning anew every month).

Very commonly a calendar includes more than one type of cycle, or has both cyclic and acyclic elements. A lunisolar calendar is synchronized both to the motion of the moon and to the apparent motion of the sun; an example is the Hebrew calendar The Hebrew calendar or Jewish calendar is a lunisolar calendar used by Jews. Today, the calendar is used predominantly for Jewish religious observances. It is used to determine the dates for Jewish holidays, and also to determine appropriate public reading of Torah portions, yahrzeits (dates to commemorate the death of a relative), and daily Psalm.

Many calendars incorporate simpler calendars as elements. For example, the rules of the Hebrew calendar depend on the seven-day week cycle (a very simple calendar), so the week is one of the cycles of the Hebrew calendar. It is also common to operate two calendars simultaneously, usually providing unrelated cycles, and the result may also be considered a more complex calendar. For example, the Gregorian calendar The Gregorian calendar is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter gravissimas. The reformed calendar was adopted later that year by a handful of countries, with other countries has no inherent dependence on the seven-day week, but in Western society The Western world, also known as the West and the Occident , is a term that can have multiple meanings depending on its context (e.g., the time period, the region or social situation). Accordingly, the basic definition of what constitutes "the West" varies, expanding and contracting over time, in relation to various historical the two are used together, and calendar tools indicate both the Gregorian date and the day of week.^[2]

The week cycle is shared by various calendar systems (although the significance of special days such as Friday, Saturday, and Sunday varies). Systems of leap days usually do not affect the week cycle. The week cycle was not even interrupted when 10, 11, 12, or 13 dates were skipped when the Julian calendar was replaced by the Gregorian calendar by various countries.

Solar calendars

Days used by solar calendars

Solar calendars assign a date to each solar day Solar time is time kept or measured by the sun; and its basic division, the day, has been recognized and used since the dawn of history. The immediately visible sign of the passage of time by the sun, and the basis of its measurement, is the sun's apparent motion along the daily course that it appears to trace out in the sky from east to west. A day may consist of the period between sunrise Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight. Because atmospheric refraction causes the sun to be seen while it is still below the horizon, and sunset Sunset or sundown is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset", with a following period of night Night or nighttime is the period of time when the sun is below the horizon. The opposite of night is day . The start and end times of night vary based on factors such as season, latitude, longitude and timezone, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day Solar time is time kept or measured by the sun; and its basic division, the day, has been recognized and used since the dawn of history. The immediately visible sign of the passage of time by the sun, and the basis of its measurement, is the sun's apparent motion along the daily course that it appears to trace out in the sky from east to west. Other types of calendar may also use a solar day.

Calendar reform

There have been a number of proposals for reform of the calendar, such as the World Calendar The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930, International Fixed Calendar The International Fixed calendar is a proposal for calendar reform providing for a year of 13 months of 28 days each, with one day at the end of each year belonging to no month or week. Though it was never officially adopted in any country, it was the official calendar of the Eastman Kodak Company from 1928 to 1989 and Holocene calendar The Holocene calendar, popular term for the Holocene Era or Human Era, is a year numbering system similar to astronomical year numbering but adds 10,000, placing its first year at the start of the Human Era the approximation of the Holocene Epoch (HE, post Ice Age) for easier geological, archaeological, dendrochronological and historical dating. The United Nations The United Nations Organization or simply United Nations (UN) (Arabic: الأمم المتحدة, French: Organisation des Nations Unies, Chinese: 联合国 / 聯合國, Spanish: Organización de las Naciones Unidas, Russian: Организация Объединённых Наций) Filipino: Organisasyon ng Nagkakaisang mga Bansa is an considered adopting such a reformed calendar for a while in the 1950s, but these proposals have lost most of their popularity.

Lunar calendars

Not all calendars use the solar year as a unit. A lunar calendar A lunar calendar is a calendar that is based on cycles of the moon phase. The only widely used purely lunar calendar is the Islamic calendar or Hijri calendar, whose year always consists of 12 lunar months. A feature of a purely lunar year, on the Islamic calendar model, is that the calendar ceases to be linked to the seasons, and drifts each is one in which days are numbered within each lunar phase A lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun. One half of the lunar surface is always illuminated by the Sun , and cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. Since antiquity, astronomers have progressively refined the definition of the tropical, a purely lunar calendar quickly drifts against the seasons, which don't vary much near the equator An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass. The capitalized term Equator refers to the Earth's equator. It does, however, stay constant with respect to other phenomena, notably tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. Most places in the ocean usually experience two high tides and two low tides each day , but some locations experience only one high and one low tide each day (diurnal tide). The times. An example is the Islamic calendar. Alexander Marshack, in a controversial reading,^[3] believed that marks on a bone baton (c. 25,000 BC) represented a lunar calendar. Other marked bones may also represent lunar calendars.^[4] Similarly, Michael Rappenglueck believes that marks on a 15,000-year old cave painting represent a lunar calendar.^[5]

Lunisolar calendars

A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. An example is the Hebrew calendar which uses a 19-year cycle.

Calendar subdivisions

Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.

Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day (29 February) in leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.

Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. For example, the year in Japan is based on the reign of the current emperor: 2006 was Year 18 of the Emperor Akihito.

See Decade, Century, Millennium

Other calendar types

Arithmetic and astronomical calendars

An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.

An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.

Complete and incomplete calendars

Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not. The early Roman calendar, which had no way of designating the days of the winter months other than to lump them together as "winter", is an example of an incomplete calendar, while the Gregorian calendar is an example of a complete calendar.

Uses

The primary practical use of a calendar is to identify days: to be informed about and/or to agree on a future event and to record an event that has happened. Days may be significant for civil, religious or social reasons. For example, a calendar provides a way to determine which days are religious or civil holidays, which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also a calendar may, by identifying a day, provide other useful information about the day such as its season.

Calendars are also used to help people manage their personal schedules, time and activities, particularly when individuals have numerous work, school, and family commitments. People frequently use multiple systems, and may keep both a business and family calendar to help prevent them from overcommitting their time.

Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.

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