The Islamic Calendar
The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. It is a lunar calendar having 12 lunar months in a year of about 354 days. Because this lunar year is about 11 days shorter than the solar year, Islamic holy days, although celebrated on fixed dates in their own calendar, usually shift 11 days earlier each successive solar year.
Islamic years are also called Hijra years because the first year was the year during which the Hijra occurred—Islamic prophet Muhammad (SAW)'s emigration from Mecca to Medina. Thus each numbered year is designated either H or AH, the latter being the initials of the Latin anno Hegirae (in the year of the Hijra).
[From Wikipedia, the Free Encyclopedia as made available under the terms of the GNU Free Documentation License.]