Calendar

A calendar is a system of reckoning and ordering time beyond the period of a day in a repetitive, usually annual, cycle. A calendar's primary function is regulating and organizing human activities; the word derives from the Latin calendarium or calendra, "account book," and kalendae or "calends," the new moon and first day of the Roman month, when Romans paid their debts. Calendars may have derived from the human penchant for imposing order; however, the most efficient exploitation of natural resources implies synchronizing productive efforts with nature's cycles. Sensitivity to such cycles is biologically programmed into humans as circadian rhythms, including the twenty-four-hour cycle of sleep and wakefulness and fluctuating body temperature; and in the female menstrual cycle, which approximates a lunar period.

Calendric periodicities are traced ultimately to the Sun and the Moon. The daily apparent rising and setting of the Sun is due to the rotation of the Earth, while the annual cycle of the seasons is related to the revolution of the Earth around the Sun, and the tilt of the Earth relevant to its plane of revolution. The most commonly reckoned calendrical period beyond day and night is the synodical lunar month (the cycle of lunar phases) of 29.5 days. Incommensurability between this period and the seasonal cycle based on the solar year of 365.24 days and the need to process fractions of days in the astronomical cycles with whole-day counts have been among the most difficult challenges for calendar specialists.