Intercalation is the insertion of a period of time into a calendar. Ancient priests and astronomers would intercalate in order to make the lunar calendar fit with the solar calendar and to make what should have been seasonal festival days fit with the season. Since the lunar calendar is based on the moon's cycle and is about 29.5 days long, while the solar calendar is based on the sun's cycle and is about 365.25-days long, the difference between the two calendars is quickly noticed. Intercalation was also done to fix those calendars that were primarily solar, like those 365-days long, although the gap between "real" time and the calendar time took longer to notice.
Intercalation is also important because while years may have fractions of days in them, calendars don't.
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In the early Roman calendar, March was the first month of the year. Intercalation was done at the end of the preceding month, February. Even when January became the first month, this tradition remained. The Julian calendar intercalated an extra day every four years. The Gregorian calendar improved on this by intercalating a leap day every four years except those evenly divisible by 100, but not 400. In both the Julian and Gregorian calendars, the intercalation is between February 28 and March 1.
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