希吉拉历, also known as the Hijri calendar, is a lunar calendar used primarily in the Islamic world. It consists of 12 months in a year of 354 or 355 days, making it about 10 to 12 days shorter than the solar Gregorian calendar. 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 希吉拉历 has either 29 or 30 days, depending on the sighting of the moon. The months include Muharram, Safar, Rabi' al-Awwal, and Ramadan, which is a significant month of fasting for Muslims. The calendar is used to determine important religious events and holidays, such as Eid al-Fitr and Eid al-Adha.