The Islamic calendar or Hijri calendar (Arabic: التقويم الهجري) is a calendar. It is used to determine Islamic holidays in most of Islam. It is a lunar calendar: it uses the phases of the Moon (rather the Earth's orbit of the Sun) to count the passing of time. Its year has 12 months of 29 or 30 days, and lasts a total of 354 or 355 days. This means that each year of the Hijri calendar is about 11 days shorter than a solar calendar such as the Gregorian. Islamic years are often called hijra years, because the first year of the calendar when the hijra occurred, that is when Prophet Muhammad went from Mecca to Medina.
Two Islamic countries (Iran and Afghanistan) use a different calendar, the Solar Hijri Calendar ("SH").
The timing of the months in the Islamic calendar is determined by direct observation of the Moon. A new month can only begin after a "waxing crescent Moon" is observed shortly after sunset. The "waxing crescent Moon" is the phase of the Moon which starts right after a New Moon. A lunar month (one new moon to the next) lasts a little over 29.5 days. For convenience, a month is the Islamic calendar is either 29 days or 30 days, one after the other. So a 29-day month is followed by a 30-day month (29+30 is the same as 29.5+29.5). In most years, that means that a year of 12 calendar months last 354 days. But that "a little over" is 44 minutes every month so, to keep the calendar in step with reality, an extra day is needed nearly every third year, so these years are 355 days long.
Whether or not the first crescent moon is actually visible differs by location. For desert communities, cloud-cover is rarely a problem but that is not true for people who live in the rain forest. One option is to use the tabular Islamic calendar, which predicts the moment of waxing crescent moon assuming perfect visibility. This method is used routinely by some Islamic communities but regarded by others as just an aid to securing a successful observation.
|Months Number||Month name
|3||Rabīʿ al-Awwal||the first spring|
|4||Rabīʿ al-Thānī or Rabī’ al-Ākhir||the second spring|
|5||Jumādá al-Ūlá||the first of dry land|
|6||Jumādá al-Ākhirah||the last of dry land|
|11||Dhū al-Qa‘dah||the one of sitting|
|12||Dhū al-Ḥijjah||the one of pilgrimage (Hajj)|
- Emile Biémont, Rythmes du temps, Astronomie et calendriers, De Borck, 2000, 393p.