The Hijra ("migration"), or Hegira, was a turning point in the history of Islam in that it was the point at which the religion was systematized into a community of Muslims. More specifically, the Hijra is the point at which Muhammad left Mecca and established the world's first Islamic state at Yathrib, renamed Medina, in present-day Saudi Arabia. Importantly, Muhammad consolidated his followers by breaking ties with his former tribe in demonstration of brotherhood with his new, more expansive and more spiritual one.
Muhammad's example spread and Islam gradually developed into the strong community of devoted believers with shared beliefs and proscriptions for behavior that survives today.
So important was Hijra to the Islamic tradition that the Islamic calendar, "Hijri", starts from the year in which it occurred: 622 CE, or 1 AH. As such, Al-Hijra is also observed by Muslims as the first day of the Islamic New Year and a time to make resolutions of self-improvement.
Traditionally, the date of Al-Hijra is whichever day in Muharram — the first month of the Islamic calendar — a crescent moon is first seen with the naked eye. However, since this cannot be determined in advance with perfect accuracy, printed Islamic calendars are generally based on estimates.