In 570-571 AD, in the Arabian city of Mecca, Islamic Prophet of God Hazarat Muhammad was born in a Quarish Family. His father died before he took birth and his mother died when he was 6 year old.
The orphaned Muhammad was brought up by his uncle.
At the age of 25, he married a 40 years Old Khadijah bint Khuwaylid. She was the first Muslim to embrace (after Muhammad) Islam.
At the age of 40, he received first revelation from God. Three years later he started preaching the revelations and proclaimed that "God is one". He also proclaimed that Complete surrender to God is the only way acceptable to him and he was a "prophet and Messenger" of the God.
The beginning of these proclamations invited harsh treatment and enmity with some of the Mecca Tribes.
In 622 AD, Muhammad migrated to Medina and this event is called Hijra, marking the beginning of Islamic Calendar.
So, derived from Hijra, the calendar is called Hijri Calender.
The first day of the this calendar marks the event Hijra. The year in Islamic calendar is suffixed AH. A few events in Islam, which are prior to the event of Hijra are suffixed with BH.
The Hijri Calendar is a Lunar Calendar that has 12 Lunar Months with 354-355 days. Since in Gregorian Calendar year 622 marks the Hijra, one may deduct 622 from the current year to know the AH year.
But this calculation is wrong. This is because; the Islamic Calendar is a Lunar Calendar, while the Gregorian calendar is a Solar Calendar. There is always a drift of 11/12 days annually.
This is reason behind the fact that Muslim Festivals such as Eid don't synchronize with Seasons. The Seasonal relation is repeated in each approx. 33 years. To simply count the Hijri Year, we deduct 621.57 years and then divide the result by .97023.
For example year 2010 in Islamic Year is calculated as follows:
2010-621.57 = 1388.43 /0.97023 = 1431
The primary use of the Islamic Calendar is only for religious purposes now.
The number of followers increased and in 630 AD Muhammad returned to Mecca and established his eminence. When he died in 632 AD, Islam had started propagating.