page_title: Perpetual calendar
hierarchical_section_title: Perpetual calendar
caption_attribution_description: English: Illustration from 1881 US Patent 248872, for a perpetual calendar paperweight. The upper section is rotated to reveal one of seven lists of years (splitting leap years) for which the seven calendars below apply.
context_page_description: A perpetual calendar is a calendar valid for many years, usually designed to look up the day of the week for a given date in the future.
  For the Gregorian and Julian calendars, a perpetual calendar typically consists of one of three general variations:
  14 one-year calendars, plus a table to show which one-year calendar is to be used for any given year. These one-year calendars divide evenly into two sets of seven calendars: seven for each common year with each of the seven starting on a different day of the week, and seven for each leap year, again with each one starting on a different day of the week, totaling fourteen.
  Seven one-month calendars and one or more tables to show which calendar is used for any given month. Some perpetual calendars' tables slide against each other, so that aligning two scales with one another reveals the specific month calendar via a pointer or window mechanism. The seven calendars may be combined into one, either with 13 columns of which only seven are revealed, or with movable day-of-week names.
context_section_description: A perpetual calendar  is a calendar valid for many years, usually designed to look up the day of the week for a given date in the future.
  For the Gregorian and Julian calendars, a perpetual calendar typically consists of one of three general variations:
  14 one-year calendars, plus a table to show which one-year calendar is to be used for any given year.  These one-year calendars divide evenly into two sets of seven calendars: seven for each common year (year that does not have a February 29) with each  of the seven starting on a different day of the week, and seven for each leap year, again with each one starting on a different day of the week, totaling fourteen. (See Dominical letter for one common naming scheme for the 14 calendars.)
  Seven (31-day) one-month calendars (or seven each of 28–31 day month lengths, for a total of 28) and one or more tables to show which calendar is used for any given month.  Some perpetual calendars' tables slide against each other, so that aligning two scales with one another reveals the specific month calendar via a pointer or window mechanism. The seven calendars may be combined into one, either with 13 columns of which only seven are revealed, or with movable day-of-week names (as shown in the pocket perpetual calendar picture).
  A mixture of the above two variations - a one-year calendar in which the names of the months are fixed and the days of the week and dates are shown on movable pieces which can be swapped around as necessary.
  Such a perpetual calendar fails to indicate the dates of moveable feasts such as Easter, which are calculated based on a combination of events in the Tropical year and lunar cycles.  These issues are dealt with in great detail in Computus.
  An early example of a perpetual calendar for practical use is found in the Nürnberger Handschrift GNM 3227a. The calendar covers the period of 1390–1495 (on which grounds the manuscript is dated to c. 1389). For each year of this period, it lists the number of weeks between Christmas day and Quinquagesima. This is the first known instance of a tabular form of perpetual calendar allowing the calculation of the moveable feasts that became popular during the 15th century.
caption: Illustration from 1881 U.S. Patent 248872, for a perpetual calendar paperweight. The upper section is rotated to reveal one of seven lists of years (splitting leap years) for which the seven calendars below apply.
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