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. 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.
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."
Blank-day calendars remove a day or two from the weekday cycle, resulting in a year length of 364 weekdays. Since this number is evenly divisible by 7, every year can begin on the same weekday. In the twelve-month plan of The World Calendar, for example, the Gregorian year-end date of December 31 is sequestered from the cycle of the week and celebrated as "Worldsday." December 30 falls on a Saturday, Worldsday follows next, and then January 1 begins every new year on a Sunday. The extra day in leap year is treated similarly. Blank-day calendars with thirteen months have also been developed: The Georgian calendar, by Hirossa Ap-Iccim (=Rev. Hugh Jones) (1745); The Positivist calendar, by Auguste Comte (1849); and the International Fixed Calendar, by Moses B. Cotsworth (1902), and championed by George Eastman. Blank-day reform proposals face a religious objection, however. Sabbatarians, who are obliged to regard every seventh day as a day of rest and worship, must observe their holy day on a different weekday each year.
Leap week calendar plans often restrict common years to 364 days, or 52 weeks, and expand leap years to 371 days, or 53 weeks. The added week may extend an existing month, or it may stand alone as an inserted seven-day month. A proposed thirteen-month reform of this type is the Pax Calendar, by James A. Colligan, S.J. (1930). The twelve-month Jubilee Calendar of Cecil L. Woods (before 1955) inserts an extra week called "Jubilee" before January in specified years. The Hanke-Henry Permanent Calendar (2003) inserts an extra year-end month of seven days called "Xtra."
The Sabbatarian objection is surmounted by the leap-week proposals. But the leap-week idea faces other objections. One is related to Christians' celebration of Easter. The relatively large leap-year correction of seven days allows March 21 to drift too far from the spring equinox. A second focuses on the complexity of the rule for observing leap years. In the Pax Calendar, for example, the extra week is added in every year having its last number, or its last two numbers, divisible by 6, and in every year ending with the number 99, and every centennial year not divisible by 400. A third line of complaint targets the ambiguity and inconvenience of the periodic extra week or month for billing and payment cycles, and for dividing the leap year into halves and quarters. A fourth complaint, finally, concerns the large number of birthdays and anniversaries falling only in the leap week/month: just over five times the number now falling on February 29.
Besides blank-day and leap-week reforms only a few other options for achieving a perennial calendar have been suggested. The Long-Sabbath Calendar, by Rick McCarty (1996), extends to thirty-six hours the last Saturday of the year and the subsequent first Sunday of the new year. Seventy-two hours are thereby covered with two weekdays instead of the usual three, which shortens the year to 364 calendar days without interrupting the weekday cycle. Another option would trim every year to exactly 364 days, allowing the calendar months to drift relative to the seasons. January would move from mid-winter to mid-summer, in the northern hemisphere, after approximately 150 years. The calendar year can be reckoned to drift though all the seasons once every 294 calendar years equal to 293 years of 365.2423208... days.
- Thomas Ignatius M. Forster, Perennial calendar and companion to the almanack (London: Harding, Mavor and Lepard, 1824)
- Hirossa Ap-Iccim, "An Essay on the British Computation of Time, Coins, Weights, and Measures, and a Proposal for a New Georgian Æra, not to Err a Day in 10,000 Years," The Gentleman’s Magazine, 15 (1745): 377-379
- Moses B. Cotsworth, The rational almanac: tracing the evolution of modern almanacs from ancient ideas of time, and suggesting improvements (Acomb, England:Cotsworth, 1905)
- Frank Parker Stockbridge, "New Calendar by 1933 -- Eastman," Popular Science Monthly (June 1929): 32
- Elisabeth Achelis, "OCCASIONAL LEAP-WEEKS NOT PRACTICAL," The Journal of Calendar Reform, 25 (Dec. 1955-Jan. 1956): 187-190
- Edward L. Cohen, "Adoption and Reform of the Gregorian Calendar," in Deanna Haunsperger, Stephen Kennedy, eds., The edge of the universe: celebrating 10 years of Math horizons (Washington, D.C.: Mathematical Association of America, 2006), pp. 129-134