Islamic calendar
The Islamic calendar or Hijri calendar () is a calendar. It is used to determine Islamic holidays in most of Islam. It is a lunar calendar: it uses the phases of the Moon (rather the Earth's orbit of the Sun) to count the passing of time. Its year has 12 months of 29 or 30 days, and lasts a total of 354 or 355 days. This means that each year of the Hijri calendar is about 11 days shorter than a solar calendar such as the Gregorian. Islamic years are often called hijra years, because the first year of the calendar when the hijra occurred, that is when Prophet Muhammad went from Mecca to Medina.