A Brief Introduction to the Islamic Calendar:
The Islamic calendar is based on lunar months, which begin when the thin crescent Moon is actually sighted in the western sky after sunset a day or so after New Moon. For Islamic calendar purposes, the sighting must be made with the unaided eye.
The names of the 12 months of the Islamic calendar, transliterated into the Roman alphabet, and their formal tabular lengths are:
|Ramadân||30||Month of fasting|
|Dhú'l-Hijjab||29||Holy month; in a leap year Dhú'l-Hijjab has 30 days.|
Since 12 lunar months are, on average, 11 days shorter than the (Gregorian) civil year, the Islamic year shifts earlier in each civil year by about this amount. The count of years for the Islamic calendar begins with 1 Muharram A.H. 1 (Anno Hegira), which corresponds to Friday, 16 July 622 C.E. (Julian calendar).
Tabular Islamic Calendar:
A tabular Islamic calendar has been established for some non-religious purposes in which the lengths of the months alternate between 29 and 30 days. This calendar consists of a 30-year cycle in which 11 of the 30 years are leap years. In leap years, an extra day is added to the last month, Dhú'l-Hijjab. Civil dates corresponding to important Islamic dates in this tabular calendar are available here.
For further information on calendars, see Richards, E.G. 2012, "Calendars," from the Explanatory Supplement to the Astronomical Almanac, 3rd edition, S.E Urban and P.K. Seidelmann eds., (Mill Valley, CA: University Science Books), Chapter 15, pp. 585-624.
Related information on these web pages includes:
Phases of the Moon and Percent of the Moon Illuminated (definitions) in FAQ
Dates of Primary Phases of the Moon in Data Services
Sun or Moon Rise/Set Table for One Year in Data Services
Complete Sun and Moon Data for One Day in Data Services
Crescent Moon Visibility in FAQ