The migration that marks the beginning of the Islamic Hijri calendar era is the Hijra.
The Hijra was Muhammad’s migration from Mecca to Medina in 622 CE. The journey and the establishment of the Muslim community in Medina became the reference point for the Islamic calendar. The calendar era is often written AH, from the Latin phrase Anno Hegirae, meaning “in the year of the Hijra.”
The Islamic calendar does not begin on the exact anniversary of the journey in the Gregorian system. Its first year begins with the first day of Muharram in the year associated with the migration. Ramadan and Eid dates are then counted within this lunar calendar.
The Hijra is different from the later Conquest of Mecca in 630 CE. The migration established a new community; the conquest was a later historical event.