This list of calendars records the various historical calendars used around the world at various times.
Historical calendars are often grouped into larger categories by cultural sphere or historical period; these are the Ancient Egyptian calendars, Babylonian calendars (Ancient Mesopotamia), Indian calendars (Hindu and Buddhist traditions of the Indian subcontinent), Chinese calendars and Mesoamerican calendars. In Classical Antiquity, the Hellenic calendars inspired the Roman calendar, including the Julian calendar introduced in 45 BC.
Calendars may also be categorised by their primary units (the astronomic bodies that define them): solar calendars are governed by the revolution of Earth around the Sun (making a solar year), lunar calendars are governed by revolution of the Moon around the Earth (making a lunar month) and lunisolar calendars are determined by an algorithmic combination of both. A day in all three types is determined by duration of the rotation of the Earth around its own axis.
A year in most solar calendars begins at or near a solstice or an equinox. Because a solar year lasts for a little less than 365 days, a solar calendar year lasts 365 days in common years and the fractional days are aggregated to have intermittent leap years of 366 days. This type of calendar has twelve months of 28 to 31 days. The best known solar calendar is the Gregorian calendar, introduced in 1582 as a refinement of the (solar) Julian calendar, is the de facto global civil calendar today.
A month in a lunar calendar begins either at the first crescent visible crescent of a new moon or at the moment of maximum full moon. A lunation (time between repeats of the same lunar phase) lasts a little over 28 days, so in this case a lunar month lasts 28 or 29 days. A lunar year consists of twelve lunar months, lasts about 354 days is independent of the solar cycle and thus migrates with respect to the seasons. The lunar new year's day is twelve lunar months after the preceding one. The best known lunar calendar is the Islamic calendar (or "Lunar Hijri calendar").
Months in a Lunisolar calendar also align with repeated phases of the moon but are designed so as not too drift too far away from the rhythm of the seasons. The accumulated 11-day differences are aggregated into a 13th or leap month, to bring it back into synchronicity with the solar cycle. The lunisolar new year's day is culturally determined. Various lunisolar calendars are used traditionally in Eastern, South-eastern and Southern Asia.
All of these calendars may also use weeks, which is a culturally determined number of days shorter than a month and possibly related to the length of moon phases.
In the list below, specific calendars are given, listed by calendar type, time of introduction (if known), and the context of use and cultural or historical grouping (if applicable). Where appropriate, the regional or historical group is noted: Jewish, Muslim, Sikh, Mayan, Aztecan, Egyptian, Mesopotamian, Persian, Hindu, Buddhist, Pre-Columbian Mesoamerican, Hellenic, Julian or Gregorian-derived.
Calendars fall into four types: lunisolar, solar, lunar (as described above) and seasonal. The seasonal calendars rely on regular climatic changes in the environment (e.g., "wet season", "dry season") rather than direct lunar or solar observations. The Islamic and some Buddhist calendars are lunar, while most modern calendars are solilunar, based on either the Julian or the Gregorian calendars. The latter are often also known as solar since strictly solar calendars like the Egyptian have fallen out of use.
Some calendars listed are identical to the Gregorian calendar except for substituting regional month names or using a different calendar epoch. For example, the Thai solar calendar (introduced 1888) is the Gregorian calendar using a different epoch (543 BC) and different names for the Gregorian months (Thai names based on the signs of the zodiac).
Regional or historical names for lunations or Julian/Gregorian months