622 – The Beginning of the Islamic Calendar

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The Islamic calendar or Muslim calendar or Hijri calendar (Arabic: التقويم الهجري‎; at-taqwīm al-hijrī; Persian: تقویم هجری قمری ‎ taqwīm-e hejri-ye qamari; Turkish: Hicri Takvim) is a lunar calendar having 12 lunar months in a year of 354 or 355 days, used to date events in many Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its first year was the year during which the Hijra, the emigration of the Islamic prophet Muhammad from Mecca to Medina, occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra). A limited number of years before Hijra (BH) are used to date events related to Islam, such as the birth of Muhammad in 53 BH.

Text:wikipedia.com