The Hijra is celebrated annually on 1 Muharram , the first day of the Muslim year , causing many writers to confuse the first day of the year of the Hijra with the Hijra itself , erroneously stating that the Hijra occurred on 1 Muharram AH 1 ( i.e. 18 April 622 ) or even the hypothetical Gregorian date from retro-calculating on the Hijri calendar 16 July 622 even though it actually occurred on 12 Rabi ' I ( i.e. 27 June 622 ) .
Many writers confuse the first day of the year of the Hijra with the Hijra itself , erroneously stating that the Hijra occurred on 1 Muharram AH 1 or 16 July 622 .