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A perennial calendar is a calendar that applies to any year, keeping the same dates, weekdays and other features.
Perennial calendar systems differ from most widely used calendars which are annual calendars. Annual calendars include features particular to the year represented, and expire at the year's end. A perennial calendar differs also from a perpetual calendar, which is a tool or reference to compute the weekdays of dates for any given year, or for representing a wide range of annual calendars.
For example, most representations of the Gregorian calendar year include weekdays and are therefore annual calendars, because the weekdays of its dates vary from year to year. For this reason, proposals to perennialize the Gregorian calendar typically introduce one or another scheme for fixing its dates on the same weekdays every year.