calendar : a system for fixing the beginning, length, and divisions of the civil year and arranging days and longer divisions of time (as weeks and months) in a definite order — Webster
Calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. A date is the designation of a single, specific day within such a system. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun or the moon. Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs. A calendar is also a physical device (often paper) or computer program. — Wikipedia (List of Calendars)
Perpetual calendar is a calendar valid for many years, usually designed to allow the calculation of 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 two general variations:
(1) 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) that starts on each day of the week, and seven for each leap year that starts on each day of the week, totaling fourteen.
(2) 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. — Wikipedia
Calendar reform roperly calendrical reform, is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar. Most calendars have several rules which could be altered by reform:
Whether and how days are grouped into subdivisions such as months and weeks, and days outside those subdivisions, if any.
Which years are leap years and common years and how they differ.
Numbering of years, selection of the epoch, and the issue of year zero.
Start of the year (such as southern solstice, January 1, March 1, northward equinox, Easter).
If a week is retained, the start, length, and names of its days.
Start of the day (midnight, sunrise, noon, or sunset).
If months are retained, number, lengths, and names of months,
Special days and periods (such as leap day or intercalary day).
Alignment with social cycles.
Alignment with astronomical cycles.
Alignment with biological cycles.
Literal notation of dates. — Wikipedia
The World Calendar (The World Calendar Association)
Example of a proposal that proposes a 12-month, perennial calendar with equal quarters, and explains the advantages of this calendar.
Calendar reform (Bill Collins)
Summary of Reform Calendars (Calendar Zone)
Annotated list of sites with proposed calendars and information about calendar reform.
Here are links to pages about closely related subjects.
See also Zoom