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A 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). This is the most common usage of the word. Other similar types of calendars can include computerised systems, which can be set to remind the user of upcoming events and appointments.
As a subset, 'calendar' is also used to denote a list of particular set of planned events (for example, court calendar).

Calendar systems

A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.

The simplest calendar system just counts days from a reference day. This applies for the Julian day. Virtually the only possible variation is using a different reference day, 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, more commonly, 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

Calendars with two levels of cycles:
*year, month, and day - most systems, including the Gregorian calendar (and its very similar predecessor, the Julian calendar), the Islamic calendar, and the Hebrew calendar
*year, week, and weekday - e.g. the ISO week date

Cycles can be synchronised with periodic phenomena:

*A lunar calendar is synchronized to the motion of the Moon (lunar phases); an example is the Islamic calendar.
*A solar calendar is based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar.
*There are some calendars that appear to be synchronized to the motion of Venus, such as some of the ancient Egyptian calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator.
*The week 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 Jewish calendar.

Many calendars incorporate simpler calendars as elements. For example, the rules of the Jewish calendar depend on the seven-day week cycle (a very simple calendar), so the week is one of the cycles of the Jewish 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 has no inherent dependence on the seven-day week, but in Western society the two are used together, and calendar tools indicate both the Gregorian date and the day of week.

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.

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 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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This is an extract from Wikipedia, the Free Encyclopedia



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