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. Each month begins 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. The Hijri calendar was established in 622 CE, the year of the Hijra, when the Prophet Muhammad migrated from Mecca to Medina. This calendar is widely used in Muslim-majority countries and by Muslims around the world for religious purposes, while the Gregorian calendar is often used for civil purposes.