The Hijri Calendar, also known as the Islamic Calendar, is a lunar calendar consisting of 12 months in a year of 354 or 355 days. It is used primarily to determine the proper days of Islamic holidays and rituals, such as Ramadan and Hajj. The calendar begins with the year of the Hijra, the migration of the Prophet Muhammad from Mecca to Medina in 622 CE. Each month in the Hijri Calendar starts with the sighting of the new moon, making it about 10 to 12 days shorter than the solar calendar used in most of the world today. The months include Muharram, Safar, Rabi' al-Awwal, and others, with Dhu al-Hijjah being the last month, during which the Hajj pilgrimage occurs.