The event that marks the beginning of the Islamic calendar in 622 AD is the Hijra, the migration of the Prophet Muhammad and his followers from Mecca to Medina (then known as Yathrib). This migration was a pivotal moment in Islamic history, as it allowed Muhammad to establish a Muslim community and political state in Medina. The Hijra is significant not only for its historical importance but also for its role in shaping the Islamic faith and community. The Islamic calendar, known as the Hijri calendar, is lunar-based and began with this event, serving as a timeline for major Islamic events and celebrations.