What calendar is used by Muslims?
Islamic calendar
The Islamic, Muslim, or Hijri calendar (Arabic: التقويم الهجري‎ at-taqwīm al-hijrī) is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca. The civil calendar of almost all countries where the religion is predominantly Muslim is the Gregorian calendar. Notable exceptions to this rule are Iran and Afghanistan, which use the Solar Hijri calendar. Rents, wages etc. are generally paid by the civil calendar.
Calendar
The Islamic calendar or Hijri calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its epoch is the Hijra (corresponding to AD 622)
With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately each 33 Islamic years.