The epoch (reference date) of the Islamic era was set as the year of the emigration of Muhammad and his followers from Mecca to Medina, known as the Hijrah, which equates to 622 CE in the Gregorian calendar.
[1] All religious duties, such as prayer, fasting in the month of Ramadan, and pilgrimage, and the dates of significant events, such as celebration of holy nights and festivals, are calculated according to the Islamic calendar.
For example, the Umm al-Qura calendar used in Saudi Arabia was reformed several times in recent years.
For example, 1 Muharram 1432 was defined to correspond to 7 or 8 December 2010 in civil calendars (depending on the country).
[5] Twelver Shia Muslims believe the Islamic new year is the first of Rabi' al-Awwal rather than Muharram, due to it being the month in which the Hijrah took place.