calendars

Calendars are systems for organizing days for social, religious, commercial, and administrative purposes. They provide a continuous sequence of days and a structure for naming, grouping, and intercalating time. Common divisions include days, weeks, months, and years, and most calendars include a leap mechanism to keep the civil year in sync with the underlying astronomical cycle.

Types of calendars commonly used today fall into three broad categories. Solar calendars are designed to keep years aligned with the solar year. The Gregorian calendar, the international civil calendar, is the most widespread solar system and uses a 365-day year with leap years adding a day every four years, except centuries not divisible by 400. Lunar calendars are based on the cycles of the Moon; the Islamic calendar, for example, has months alternating between 29 and 30 days and a year of about 354 days, causing dates to drift relative to the seasons. Lunisolar calendars, such as the Hebrew and Chinese systems, track lunar months but insert leap months to realign with the solar year, producing 12 or 13 months in a year.

Beyond structure, calendars include era designations for counting years, such as Anno Domini/Common Era or BCE/CE. They also organize time into weeks, with seven-day cycles in many cultures. Usage spans civil timekeeping, religious observances, cultural festivals, and fiscal year planning.

Historically, calendars emerged from ancient astronomy and agricultural needs, advancing from early lunar and solar systems to sophisticated schemes like the Julian and Gregorian reforms that shaped modern civil timekeeping.