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Islamic calendar

The Islamic calendar is the official calendar in Kuwait, Saudi Arabia, Yemen, and some other Middle Eastern countries, and is used privately by Moslems everywhere.
Era

Its era begins with the migration from Mecca to Medina of Mohammed, the Prophet of Islam, an event known as the Hegira. The initials A.H. before a date mean “anno Hegirae” or “after Hegira.” The first day of the year is fixed in the Quran as the first day of the month of Muharram. In a.h. 17 ‘Umar I, the second caliph, established the beginning of the era of the Hegira (1 Muharram a.h. 1) as the date that is 16 July 622 ce in the Julian calendar. Origin of the era. (Astronomers, however, use an “astronomical Hegira epoch” that began on 15 July 622 (Julian calendar).)
Years

The years are lunar and consist of 12 lunar months. There is no intercalary period, since the Quran (Sura IX, verses 36–37) sets the calendar year at 12 months. Because the year in the Islamic calendar is shorter than a solar year, the months drift with respect to the seasons, in a cycle 32½ years long.
Months

A new month begins when the ulama first sight the lunar crescent after a new moon. If poor visibility makes it impossible to see the moon, the new month begins 30 days after the last one began. The result is that dates may differ from city to city. In modern times, the practice of sighting the moon is observed most strictly for Ramadan, the month of fasting.

Day and week

The day begins and ends at sunset, at the instant at which a white thread can no longer be distinguished from a black one. The week has 7 days and begins at sunset on Saturday (Gregorian calendar). The names of the days of the week are numbers. The fifth day, which begins at sunset on Thursday, is the day for congregational prayer, but is not considered a day of rest.
Modern usage

Because local determination of dates can be extremely confusing, for historical and civil purposes a version of the calendar that is not based on local month-by-month observation is widely used. In this calendar, months are alternately 30 and 29 days long, making a 354-day year. Since the synodic month is not a whole number of days (and not 29½ days either), to keep the calendar month in sync with the synodic month leap days must be introduced, just as in the Gregorian calendar leap days are introduced to keep the calendar year in sync with the tropical year. These corrections bring the average length of the month to just 2.9 seconds less than the synodic month. If leap days were not added, the beginnings of months would soon not coincide with the first sighting of the lunar crescent.