<P> 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 determine when to start planting or harvesting, 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 . </P> <P> 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 . </P> <P> 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, timekeepers can show time, date and weekday . </P> <P> The Gregorian calendar is the de facto international standard, and is used almost everywhere in the world for civil purposes . It is a purely solar calendar, with a cycle of leap days in a 400 - year cycle designed to keep the duration of the year aligned with the solar year . </P>

Where did the traditional school calendar come from