Intercalation

Intercalation is the insertion of an extra day or month into a calendar to keep its dates aligned with an astronomical cycle.

Leap year: A calendar year containing an extra day or, in some calendars, an extra month. A leap year applies intercalation at the scale of a whole year.
Lunar month: The time between successive phases of the Moon, averaging about 29.53 days. Its length does not divide evenly into a solar year, motivating leap months.
Hebrew calendar: A lunisolar calendar used in Judaism, with seven leap months in each 19-year cycle. Its fixed cycle is a prominent example of systematic month intercalation.
Roman calendar: The calendar system used in ancient Rome before and during the transition to the Julian calendar. Its irregular intercalations made calendar dates vulnerable to political manipulation.
Easter computus: The method used to calculate the date of Easter in Christian calendars. Its date depends on calendar rules coordinating the solar year and lunar phases.
Leap day: An extra calendar day inserted to keep a calendar aligned with an astronomical cycle. This is the best-known form of intercalation in the Gregorian calendar.
Tropical year: The average interval between successive vernal equinoxes, about 365.24 days. It supplies the seasonal cycle that intercalation keeps a calendar near.
Chinese calendar: A lunisolar calendar that uses leap months to keep lunar months aligned with the solar year. Its leap-month rules illustrate intercalation through solar-term calculations.
Julian calendar: A solar calendar introduced by Julius Caesar, assigning a leap day every fourth year. It replaced irregular Roman adjustments with a predictable intercalation cycle.
Passover: A Jewish festival observed in the Hebrew month of Nisan, whose calendar includes leap months. Intercalation helps keep Passover in its intended spring season.

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