Hijra is the day the Holy Prophet (S.A.W.) left Mecca for Medina. In particular, it was the Prophet's (S.A.W.) abandoning Mecca because of its mounting hostility, and transferring himself and his followers to Yathrib (200 miles north) whose people had invited him. He arrived on the 20th of September 622 AD, and the city proudly changed its name to Medinatu'l-Nabi (the Prophet's (S.A.W.) city). On Imam Ali's (A.S.) advice, Omar, reproached for not dating documents, took this event as the start o f the Muslim era, dubbing the year of the Hijrah year as Year 1 and starting it on the Lunar New Year's Day, the 1st of Muharram 622 AD.