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A perennial calendar is a representation of the year abstracting from features peculiar to any particular year. Its opposite is an annual calendar, which includes features peculiar to the year represented. An annual calendar therefore expires at year's end. Representations of the Gregorian calendar year that include weekdays are 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. A perennial calendar differs also from a perpetual calendar, which is a device for computing the weekdays of dates for any given year, or for representing a wide range of annual calendars. == History and background == The term ''perennial calendar'' appeared as early as 1824, in the title of Thomas Ignatius Maria Forster's ''Perennial calendar and companion to the almanack.''〔Thomas Ignatius M. Forster, Perennial calendar and companion to the almanack (London: Harding, Mavor and Lepard, 1824)〕 In that work he compiled "the events of every day in the year, as connected with history, chronology, botany, natural history, astronomy, popular customes and antiquities, with useful rules of health, observations on the weather, explanations of the feasts and fesitivals of the church and other miscellaneous useful information." The data listed there for each date in the calendar apply in every year, and supplement data to be found in annual almanacs. Often printed in perennial-calendar format also are book blanks for diaries, ledgers and logs, for use in any year. Entries on the blank pages of these books are organized by calendar dates, without reference to weekdays or year numbers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Perennial calendar」の詳細全文を読む スポンサード リンク
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