The formal beginning of the Muslim era was chosen to be the Hijra in 622 CE, which was an important turning point in Muhammad's fortunes. The assignment of this year as the year 1 AH (Anno Hegirae) in the Islamic calendar was reportedly made by Caliph Umar. It is a lunar calendar, with nineteen ordinary years of 354 days and eleven leap years of 355 days in a thirty-year cycle. Islamic dates cannot be converted to CE/AD dates simply by adding 622 years: allowance must also be made for the fact that each Hijri century corresponds to only 97 years in the Christian calendar.[152]
The year 1428 AH coincides almost completely with 2007 CE.
Islamic holy days fall on fixed dates of the lunar calendar, which means that they occur in different seasons in different years in the Gregorian calendar. The most important Islamic festivals are Eid al-Fitr (Arabic: عيد الفطر) on the 1st of Shawwal, marking the end of the fasting month Ramadan, and Eid al-Adha (Arabic: عيد الأضحى) on the 10th of Dhu al-Hijjah, coinciding with the pilgrimage to Mecca.[153] Similar to the Jewish calendar, days in the Islamic calendar last from sunset to sunset.
(http://upload.wikimedia.org/wikipedia/commons/thumb/b/ba/Lunar_libration_with_phase_Oct_2007_450px.gif/170px-Lunar_libration_with_phase_Oct_2007_450px.gif)
The Islamic calender is based on the lunar cycle
For more details:
http://en.wikipedia.org/wiki/Islamic_calendar
(http://a8.sphotos.ak.fbcdn.net/hphotos-ak-snc7/s480x480/400488_377255912342934_1266850556_n.jpg)