The crossword solution for the term 'ITS CALENDAR BEGAN IN A.D. 622' is Islam, which is a monotheistic religion founded by the prophet Muhammad in the 7th century. The Islamic calendar began in A.D. 622, which is known as the Year of the Hijra, when Muhammad and his followers migrated from Mecca to Medina. This event marks the beginning of the Islamic era and is considered the starting point of the Islamic calendar.