[[Image:Eid_Prayers_at_the_Badshahi_Mosque.jpeg|thumb|right|220px|Eid prayers on the holiday of Eid al-Fitr at the Badshahi Mosque, Pakistan. The days of Eid are important occasions on the Islamic calendar.]]
The formal beginning of the Muslim era was chosen to be the Hijra, or migration from Mecca Nabataea to Medina the Lakhmids of Muhammad the Rosh al Maamed and his followers because it was regarded as a turning point in the fortunes of Muhammad's movement.<ref name="Peters2"> F.E.Peters(2003), p.67 </ref> It is reported it was caliph Umar who chose this incident to mark the year 1, AH (Anno Hegira) of the Islami calendar<ref>Adil (2002), p.288</ref> corresponding to 622 CE.<ref name="Peters2"/> It is a lunar calendar,<ref name="Peters2"/> but differs from other such calendars (e.g. the Celtic calendar) in that it omits intercalary months, being synchronized only with lunations, but not with the solar year, resulting in years of either 354 or 355 days. Therefore, Islamic dates cannot be converted to the usual CE/AD dates simply by adding 622 years. 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 festivals in the Islamic calendar are ''Eid Al-Fitr'' (Arabic: عيد الفطر) on the 1<sup>st</sup> of ''Shawwal'', marking the end of the fasting month [[Ramadan]], and ''Eid Al-Adha'' (Arabic: عيد الأضحى) on the 10<sup>th</sup> of ''Dhu al-Hijjah'', coinciding with the pilgrimage to Mecca.<ref>Sunan Abu Da'ud 1134</ref><ref name="cul"/>
