Perennial calendar

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.

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. 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 festivals 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.

A goal of modern calendar reform has been to achieve universal acceptance of a perennial calendar, with dates fixed always on the same weekdays, so the same calendar table serves year after year. Advantages claimed for a perennial over an annualized calendar like the Gregorian are simplicity and regularity. Scheduling is simplified for institutions and industries with extended production cycles. School terms and breaks, for example, can fall annually on the same dates. Month-based ordinal dating ("Fourth Thursday in November", "Last Monday in May") will be obsolete. Two methods favored for perennializing the calendar are the introduction of so-called "blank days," and of a periodic "leap week."

...
Wikipedia