Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A lunar calendar is a calendar that is based on cycles of the moon phase. The only widely used purely lunar calendar is the Islamic calendar or Hijri calendar, whose year always consists of 12 lunar months. A feature of a purely lunar year, on the Islamic calendar model, is that the calendar ceases to be linked to the seasons, and drifts each solar year by 11 to 12 days, and comes back to the position it had in relation to the solar year approximately every 33 Islamic years. It is used predominantly for religious purposes. In Saudi Arabia it is also used for commercial purposes.