The Hegira was instituted by Omar, the second
caliph, in imitation of the aera of the martyrs of the
Christians, (D'Herbelot, p. 444;) and properly commenced
sixty-eight days before the flight of Mahomet, with the
first of Moharren, or first day of that Arabian year which
coincides with Friday, July 16th, A.D. 622, (Abulfeda, Vit
Moham, c. 22, 23, p. 45 - 50; and Greaves's edition of Ullug
Beig's Epochae Arabum, etc., c. 1, p. 8, 10, etc.)