Ancient civilizations created calendars based on observable events such as the seasons and phases of the Moon. A lunar calendar is based on the monthly cycle of the Moon's phases. An ancient Roman priest watched the sky and announced every new moon to the king. The new moon signaled the start of a new month. The first day of the month was called a 'Kalends' based on their word 'calare,' which means 'to announce solemnly.' The modern word 'calendar' evolved from Roman customs.


1. Intercalation

Most lunar calendars are actually lunisolar calendars. A lunisolar calendar determines months with the lunar cycle and then adds intercalations. Intercalation is the process of inserting leap days, weeks, or months to align a lunar calendar with the solar year or seasons. The earliest known solar calendar was used in ancient Egypt. Evidence suggests that it was based on a lunar calendar. The solar calendar was used for civic and agricultural purposes, while the lunar calendar remained in use for religious purposes.

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