A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. The term calendar can also refer to a physical record (often a paper document) of such a system. Calendars can be based on astronomical events, such as the movement of the sun or the moon, or they can be based on arbitrary conventions, such as the Gregorian calendar, which is the most widely used calendar in the world today.
The Gregorian calendar is a solar calendar that was introduced in 1582 by Pope Gregory XIII. It is based on the Earth’s orbit around the sun, and it consists of 365 days in a year, with an extra day added every four years (except for years that are divisible by 100 but not by 400). The Gregorian calendar is the most accurate calendar in use today, and it is the basis for most of the world’s civil calendars.
