The Islamic calendar, also known as the Hijri calendar or the lunar calendar, is a calendar system based on the cycles of the moon. The Islamic calendar has 12 lunar months, and each month is either 29 or 30 days long. A new month starts when the first crescent of the new moon is sighted.

The Islamic calendar is used to determine the dates of Islamic holidays and events, including Ramadan, Eid al-Fitr, and Eid al-Adha. The first month of the Islamic calendar is Muharram, and the Islamic year begins with the first day of Muharram.

The Islamic calendar is approximately 11 days shorter than the Gregorian calendar, which is the calendar system used in most parts of the world. This means that the dates of Islamic holidays and events vary each year in relation to the Gregorian calendar. For example, Ramadan, which is the ninth month of the Islamic calendar, occurs approximately 11 days earlier each year in relation to the Gregorian calendar.