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Intercalation, also known as intercalary insertion, is a process in which one or more days, months, or years are added to a calendar to synchronize it with the seasons or other astronomical events. This practice is commonly used in various cultures and calendars around the world to maintain the alignment of the calendar with the solar year.

In the Gregorian calendar, which is widely used in many countries, an intercalary day is added to February every four years to account for the extra time it takes for the Earth to complete one orbit around the Sun. This day is known as a leap day and occurs on February 29 in a leap year.

In some cultures, intercalation is used to adjust the calendar for religious or cultural purposes. For example, in the Hebrew calendar, an intercalary month is added seven times every 19 years to keep the calendar in sync with the lunar cycle.

Intercalation can also be used to correct errors in the calendar. For instance, in the Julian calendar, which was used before the Gregorian calendar, an intercalary day was added every four years to correct for the discrepancy between the solar year and the calendar year. However, this correction was not precise enough, leading to the need for the Gregorian calendar.

Intercalation is an important concept in the study of calendars and timekeeping, as it allows for the synchronization of the calendar with astronomical events and the maintenance of cultural and religious traditions.