The Hajj is an annual Islamic pilgrimage to Mecca, the holiest city for Muslims, and a mandatory religious duty for Muslims that must be carried out at least once in their lifetime by all adult Muslims who are physically and financially capable of undertaking the journey, and can support their family during their absence. The Hajj is the most important and the second largest annual gathering of Muslims in the world. The largest gatherings take place on Ashoura and Arbaeen of Imam Hossein (AS), the 3rd Imam in Shiite Islam.