Calendar reform or 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 design.
Principles
The prime objective of a calendar is to unambiguously identify any day in past, present and future by a specific date in order to record or organize social, religious, commercial or administrative events. Recurring periods that contain multiple days, such as weeks, months, and years, are secondary, convenient features of a calendar. Most cultures adopt a primary dating system, but different cultures have always needed to align multiple calendars with each other, either because they coexisted in the same space (e.g. secular and religious groups with different demands) or had established trading relations.
Once specified, a calendar design cannot be altered without becoming a new design. If a proposed design is sufficiently close to the legacy one, i.e. compatible with it, a reform of the local calendar system is possible without disruption. Examples of this include the changes to the Chinese calendar due to problems between regions in China and practical changes in religious calendars such as the Islamic calendar, where the start of the month is now derived from astronomical data rather than sightings by religious leaders.
Some design changes, however, will yield date identifiers different from the previous design for some days, often in the distant past or future. The calendar system must clarify whether dates are changed to the new design retroactively (using a proleptic calendar) or whether the design in use then and there shall be respected. Calendar schisms happen if not all cultures that adopted a common calendar system before perform a calendar reform at the same time. If date identifiers are similar but different, confusion and mistakes are unavoidable.
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 the December solstice, January 1, March 1, March equinox, Lady Day). 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.
Historical reforms
Historically, most calendar reforms have been made in order to synchronize the calendar with the astronomical year (either solar or sidereal) and/or the synodic month in lunar or lunisolar calendars. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar, when it was altered to the Gregorian calendar.
The fundamental problems of the calendar are that the astronomical year has neither a whole number of days nor a whole number of lunar months; neither does the lunar month have a whole number of days: in each case there are fractions "left over". (In some physical circumstances the rotations and revolutions of a planet and its satellite(s) can be phase-locked —for example the same side of the moon always faces us— but this has not operated to lock together the lengths of the Earth's year, day and month.) Such remainders could accumulate from one period to the next, thereby driving the cycles out of synchronization.
A typical solution to force synchronization is intercalation. This means occasionally adding an extra day (or month) into the cycle. An alternative approach is to ignore the mismatch and simply let the cycles continue to drift apart. The general approaches include: The lunar calendar, which fits days into the cycle of lunar months, adding an extra day when needed, while ignoring the annual solar cycle of the seasons. The solar calendar, which fits artificial months into the year, adding an extra day into one month when needed, while ignoring the lunar cycle of new/full moons. The lunisolar calendar, which keeps both the lunar and solar cycles, adding an extra month into the year when needed.