The Islamic calendar (Arabic: التقويم الهجري) is a calendar. It is used to determine islamic holidays. It is a lunar calendar. It has 12 months, and about 354 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 Muhammad went from Mecca to Medina. Each year is therefore labelled with either H or AH.