Historical; not practised continuously

A calendar is a system for organising and counting days into larger units (weeks, months, years and eras), and this category holds the calendars of the world's traditions: solar, lunar and lunisolar, with their months, intercalations and reckonings of the year. Each entry records a system's structure, where and by whom it is used, and how it relates to the common civil calendar.

The fundamental problem every calendar solves is that the natural cycles do not divide evenly: the year is not a whole number of days or of lunar months, so any calendar must choose which cycle to follow and how to reconcile it with the others.

A solar calendar (like the Gregorian) follows the Sun and keeps the seasons fixed but lets the months drift from the Moon; a lunar calendar (like the Islamic) follows the Moon and lets the year drift through the seasons; a lunisolar calendar (like the Hebrew, Chinese and Hindu) follows the Moon but adds a leap month periodically to stay in step with the Sun. Each choice shapes a tradition's festivals and life.

The atlas describes each calendar's rule, its months and eras, and its religious use. The individual months and units of particular calendars (Hebrew, Islamic, Maya and others), are treated in their own categories under Calendars & Time.