The Islamic calendar or Muslim calendar, also called the Hijri calendar) is the 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. 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, such as a year of the Gregorian calendar. Islamic years are also called Hijra years because the first year was the year during which the Hijra occurred - Muhammad'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.)

The predecessor to the Islamic calendar was a lunisolar calendar which used lunar months, but was also synchronized with the seasons by the insertion of an additional, intercalary month, when required. Whether the intercalary month (Nasi) was added in the spring like that of the Hebrew calendar or in autumn is debatable. It is assumed that the intercalary month was added between the twelfth month (the month of the pre-Islamic Hajj) and the first month (Muharram) of this pre-Islamic year.

The two Rabi' months denote grazing and the modern Meccan rainy season (only slightly less arid than normal), which would promote the growth of grasses for grazing, occurs during autumn. These imply a pre-Islamic year beginning near the autumnal equinox. However, the rainy season after which these months are named may have been different when the names originated (before Muhammad's time) or the calendar may have been imported from another region which did have such a rainy season. On the other hand, Muhammad forbade the intercalary month (released the calendar from the seasons) near the end of his life, which implies a pre-Islamic year beginning near the vernal equinox because that is when the modern lunar year began during his last year.

Abraha, a governor of Yemen, then a province of the Christian Kingdom of Aksum (modern Ethiopia), attempted to destroy the Kaaba with an army which included an elephant (possibly several). Although the raid was unsuccessful, because it was customary to name a year after a major event which occurred during it, that year became known as the Year of the Elephant, which was also the year that Muhammad was born.

Although most Muslims equate it with the Western year 570, a minority equate it with 571. Later years were numbered from the Year of the Elephant, whether for the years of the pre-Islamic lunisolar calendar, the lunisolar calendar used by Muhammad before he forbade the intercalary month, or the first few years of the lunar calendar thus created.

In 638 (AH 17), the second Caliph Umar began numbering the years of the Islamic calendar from the year of the Hijra, which was postdated AH 1. The first day of the first month (1 Muharram) of that proleptic Islamic year, that is, after the removal of all intercalary months between the Hijra and Muhammad's prohibition of them nine years later, corresponded to July 16, 622 (the actual emigration took place in September). The first surviving attested use of the Hijri calendar is on a papyrus from Egypt in 22 AH, PERF 558.

Each month has either 29 or 30 days, but usually in no discernible order. Traditionally, the first day of each month was the day (beginning at sunset) of the first sighting of the lunar crescent (the hilal) shortly after sunset. If the hilal was not observed immediately after the 29th day of a month, either because clouds blocked its view or because the western sky was still too bright when the moon set, then the day that began at that sunset was the 30Such a sighting had to be made by one or more trustworthy men testifying before a committee of Muslim leaders. Determining the most likely day that the hilal could be observed was a motivation for Muslim interest in astronomy, which put Islam in the forefront of that science for many centuries. This traditional practice is still followed in a few parts of the world, like Pakistan and Jordan. However, in most Muslim countries astronomical rules are followed which allow the calendar to be determined in advance, which is not the case using the traditional method.

Malaysia, Indonesia, and a few others begin each month at sunset on the first day that the moon sets after the sun (moonset after sunset). In Egypt, the month begins at sunset on the first day that the moon sets at least five minutes after the sun.The official Umm al-Qura calendar of Saudi Arabia used a substantially different astronomical method until recent years.

Before AH 1420 (before April 18, 1999), if the moon's age at sunset in Riyad was at least 12 hours, then the day ending at that sunset was the first day of the month. This often caused the Saudis to celebrate holy days one or even two days before other predominantly Muslim countries, including the dates for the Hajj, which can only be dated using Saudi dates because it is performed in Mecca. During one memorable year during the AH 1380s (the 1970s), different Muslim countries ended the fast of Ramadan on each of four successive days. The celebrations became more uniform beginning in AH 1420.

For AH 1420-22, if moonset occurred after sunset at Mecca, then the day beginning at that sunset was the first day of a Saudi month, essentially the same rule used by Malaysia, Indonesia, and others (except for the location from which the hilal was observed). Since the beginning of AH 1423 (March 16, 2002), the rule has been clarified a little by requiring the geocentric conjunction of the sun and moon to occur before sunset, in addition to requiring moonset to occur after sunset at Mecca.

This ensures that the moon has moved past the sun by sunset, even though the sky may still be too bright immediately before moonset to actually see the crescent. Strictly speaking, the Umm al-Qura calendar is intended for civil purposes only. Their makers are well aware of the fact that the first visual sighting of the lunar crescent can occur up to two days after the date calculated in the Umm al-Qura calendar.

Since AH 1419 (1998/99) several official sighting committees have been set up by the government of Saudi Arabia to determine the first visual sighting of the lunar crescent at the begin of each lunar month. Nevertheless, the religious authorities of Saudi Arabia also allow the testimony of less experienced observers and thus often announces the sighting of the lunar crescent on a date when none of the official committees could see the lunar crescent.

In nearly all of these cases, a retrospective analysis indicates that these extremely early reports of the lunar crescent are impossible and are based on false sightings.The moon sets progressively later than the sun for locations further west, thus western Muslim countries are more likely to celebrate some holy day one day earlier than eastern Muslim countries.There exists a variation of the Islamic calendar known as the tabular Islamic calendar in which months are worked out by arithmetic rules rather than by observation or astronomical calculation.

It has a 30-year cycle with 11 leap years of 355 days and 19 years of 354 days. In the long term, it is accurate to one day in about 2500 years. It also deviates up to about 1 or 2 days in the short term.

Microsoft uses the "Kuwaiti algorithm" to convert Gregorian dates to the Islamic ones. It is claimed to be based on a statistical analysis of historical data from Kuwait but it is in fact a variant of the tabular Islamic calendar.

Islamic Calendar Wikipedia