The term calendar is taken from calendae, the term for the first day of the month in the Roman calendar, related to the verb calare 'to call out', referring to the "calling" of the new moon when it was first seen. Latin calendarium meant

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The course of the sun and the moon are the most salient natural, regularly recurring events useful for timekeeping, thus in pre-modern societies worldwide lunation and the yearwere

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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

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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

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Arithmetical and astronomical
An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar

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The primary practical use of a calendar is to identify days: to be informed about or to agree on a future event and to record an event that has happened. Days may be significant for agricultural, civil, religious, or social reasons. For example, a calendar provides a way to

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The term calendar applies not only to a given scheme of timekeeping but also to a specific record or device displaying such a scheme, for example, an appointment book in the form of a pocket calendar (or personal organizer), desktop calendar, a wall calendar,

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