Important note: The Islamic day and month displayed here are approximate. The start of a month in the canonical Islamic calendar is based on the first physical sighting of the new moon at a given location, which depends on a number of factors which are difficult or impossible to compute, including weather and visual acuity of the observer. The date displayed above is computed based on the actual time of the new moon, which may occur a day or so before it is actually visible. Also, note that this date is computed relative to Universal Time, which is essentially equivalent to Greenwich Mean Time.
The starting point for the Islamic calendar was fixed at the date of the new moon during the first lunar month in the year in which Mohammed and his followers left Mecca for Medina (the Hegira). The abbreviation A.H. stands for 'Anno Hegirae' (Latin for the Year of the Hegira). The Hegira (which is actually pronounced Hijra) is often translated as 'the flight'; however this translation has been questioned by Islamic scholars; it probably means 'to break off from the relations or abandon one's own tribe.' The Hegira is believed to have occured on 20 September 622 C.E.
The Islamic calendar was created in 639 C.E. (A.H. 17) by the Second Caliph 'Umar ibn Al-Khattab (592-644 C.E.). The civil epoch (the zero date for the Islamic calendar) is July 16, 662 C.E., when the new moon was first visible in Arabia in the first lunar month (it actually became new the day before).
The day begins at sunset in the Islamic calendar.
The Islamic calendar is strictly based on lunar cycles. For this reason, each year is about 11 days short of a solar year. Hence the start of each month will be different from one year to the next. The Islamic calendar is used to determine important religious holidays such as the start and end of Ramadan. Therefore the date of these festivals rotate backwards through the solar year. This has been attributed to the fact that the Islamic calendar originated in the desert regions close to the equator. Seasonal differences are not as marked as in higher latitudes. Therefore it was less important that holidays synchronize with the solar year.