Islamic Calendar

The Islamic calendar (or Hijri calendar) is a purely lunar-based calendar. It contains 12 months that are based on the motion of the moon, and because 12 synodic months is only 12 x 29.53=354.36 days, the Islamic calendar is consistently shorter than a tropical year, and therefore it shifts with respect to the Christian calendar. The calendar is based on the Qur'an (Sura IX, 36-37) and its proper observance is a sacred duty for Muslims. The Islamic calendar is the official calendar in countries around the Gulf, especially Saudi Arabia. But other Muslim countries use the Gregorian calendar for civil purposes and only turn to the Islamic calendar for religious purposes.

The names of the 12 months that comprise the Islamic year are as follows:

(Due to different transliterations of the Arabic alphabet, other spellings of the months are possible.)
Each month starts when the lunar crescent is first seen (by a human observer's eye) after a new moon. Although new moons may be calculated quite precisely, the actual visibility of the crescent is much more difficult to predict. It depends on factors such as weather, the optical properties of the atmosphere, and the location of the observer. It is therefore very difficult to give accurate information in advance about when a new month will start. Furthermore, some Muslims depend on a local sighting of the moon, whereas others depend on a sighting by authorities somewhere in the Muslim world. Both are valid Islamic practices, but they may lead to different starting days for the months.

An Islamic calendar overtakes a Gregorian calendar when as the year in the Islamic calendar is about 11 days shorter than the year in the Christian calendar, the Islamic years are slowly gaining in on the Christian years. But it will be many years before the two coincide. The 1st day of the 5th month of C.E. 20874 in the Gregorian calendar will also be (approximately) the 1st day of the 5th month of AH 20874 of the Islamic calendar.