The Islamic calendar, also known as the Hijri calendar, is a lunar calendar consisting of 12 months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the Hajj.
The calendar begins with the Hijra, the migration of the Prophet Muhammad (ﷺ) from Mecca to Medina in 622 CE. This event marks the beginning of the Islamic era.
Each month begins with the sighting of the new moon. The months alternate between 29 and 30 days, with the actual length depending on the lunar cycle. This creates a calendar that is approximately 11 days shorter than the solar year.