A lunar calendar is a calendar based on the cycles of the moon's phases. The lunar calendar is different from the more commonly used solar calendar, which is based on the position of the Earth in relation to the sun.

In a lunar calendar, the months are based on the phases of the moon, with each month corresponding to one complete lunar cycle, or lunation, which is the time it takes for the moon to go through all of its phases. This typically results in months that are around 29.5 days long.

Lunar calendars are used in various cultures and religions for determining the dates of religious festivals, agricultural activities, and other traditional events. For example, the Islamic calendar is a lunar calendar used for determining the dates of Islamic holidays such as Ramadan and Eid al-Fitr.

One challenge of using a lunar calendar is that it does not perfectly align with the solar year, which can lead to discrepancies between the lunar calendar and the seasons. As a result, some lunar calendars incorporate adjustments to stay synchronized with the solar year, such as adding leap months or intercalary days.