According to Shi'a Muslims, this is the date of Muhammad's birth in Mecca.
Raised by his uncle, Muhammad will have a number of different jobs before going to the mountains where, Muslims believe, he will receive a revelation from God via the angel Gabriel. Three years after the revelations Muhammad is believed to have started preaching about them in order to convince the people of Mecca to convert and follow the one true God.
This religious proselytizing is greeted with more hostility than tolerance, forcing Muhammad to move to Medina with a few followers to escape persecution. There his number of followers increases greatly and he is able to unite several warring tribes around a common religion. After eight years of conflict he and his followers defeat the armies of Mecca and Muhammad is able to return, bringing even more Arab tribes under the banner of Islam.
Based in Mecca, the Muslim forces spread throughout Arabia, conquering and incorporating more and more tribes into a growing Islamic empire. It's possible that as many submitted to Muhammad and adopted Islam just to avoid military conflict and benefit from the spoils of war as did after military conquest.