The Islamic Calendar

The Islamic (or Hijri) calendar consists of twelve months and is lunar, with each month beginning when the first crescent after the new moon is sighted. The first crescent can be sighted either locally, or elsewhere in the world. Because the moon's cycle is either 29 or 30 days long, a year in the Islamic calendar is either 354 or 355 days long, around 10 days shorter than the Gregorian calendar. This means that the Islamic months are shifting slowly through the seasons, in cycles lasting approximately 33 years. Read more here