The Islamic calendar () is a calendar. It is used to determine islamic holidays in most of Islam. It is a lunar calendar. It has 12 months, and 354 or 355 days. This means that each year, the calendar is about 11 days shorter than a solar calendar, like 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.