Caliph `Umar bin Al-Khattab was the first to initiate the system of the lunar calendar taking the Prophet’s Hijrah from Mecca to Medina on 622 A.D as the starting event.
Later on, the Hijri calendar was referred to as (A.H.) in Western languages, which indicates the Latin Anno Hegirae.
The exact date of starting the Hijri calendar was Muharram 1,/ July 16, 622 A.D.
A Hijri (Islamic) year consists of 12 lunar months order as follows:
(Muharram, Safar, Rabi` Al-Awwal, Rabi` Al-Akhar, Jumada Al-Awwal, Jumada Al-Akhar, Rajab, Sha`ban, Ramadan, Shawwal, Dhul-Qa`dah, Dhul-Hijjah).
The Hijri calendar includes Islamic festivities such as the New Hijri year on the first of Muharram, the Isra’ and Mi`raj [Night Journey and Ascension] on the twenty-seventh of Rajab, Laitat Al-Qadr [the Night of Revelation] on the last ten days of Ramadan, `Id Al-Fitr [the Lesser Bairam] on the first of Shawwal, `Id Al-Adha [the Greater Bairam] on the tenth of Dhul-Hijjah and the Hajj season from Dhul-Hijjah 8 to 13.
N.B.
*As the Hijri calendar basically relies on the lunar cycle, the Hijri year is 11 days less than the Gregorian year.
*The lunar months of the Hijri calendar are not related to the four seasons. This means that the Islamic festivities that fall on the same month every year may occur in different seasons. Hajj and Fasting of Ramadan, for example, may occur equally in summer or in winter.
*Islamic festivities occur in the same season each 33 years when the lunar cycle is complete.
*There are slight differences between the printed Islamic calendars, because of the following:
1. There are no international criteria for determining the new crescent soon after its appearance.
2. There are different calculating methods used to determine the appearance of the new crescent.
3. There may be different weather conditions in the location of sighting the new crescent.
For the above-mentioned reasons, there is no calculation method with a 100% accuracy. Sighting the new crescent still formulates the basic method of determining certain Islamic festivities such as the advent of the Month of Ramadan and the Two `Ids.
