The Islamic calendar has its starting point at the date of the flight of Mohammed from Mecca to Medina, known as the Hejira.The most widely accepted date for this event in the Gregorian calendar is sunset at July 16, 622 AD.This dating system is used in the Muslim world (except Turkey, which uses the Gregorian calendar) and based on a year of 12 months, each month beginning approximately at the time of the New Moon. (The Iranian calendar,however, is based on a solar year.)
The Islamic calendar is tied to the lunar phase cycle, with each month alternatively having either 29 or 30 days, except for the 12th, Dhu al-Hijjah, the length of which is varied in a 30-year cycle intended to keep the calendar in step with the true phases of the Moon. The calendar therefore drifts by days relative to the sun each Islamic year, and would require Islamic years to get back in synchronization. As a result, the calendar uses an 11-year leap year cycle. In 11 years of this cycle, Dhu al-Hijjah has 30 days, and in the other 19 years it has 29. Thus, the year has either 354 or 355 days.
No months are ever added, so that the named months do not remain in the same seasons but retrogress through the entire solar, or seasonal, year (of about 365.25 days) every 32.5 solar years. The names of the Islamic months are Muharram, Safar, Rabia I, Rabia II, Jumada I, Jumada II, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Qada, and Dhu al-Hijah.