The constitution grants freedom of religion and officially makes Bangladesh a secular state, while establishing Islam as the "state religion of the Republic". Islam is followed by 90 percent of the population. Most Bangladeshis are Bengali Muslims, who form the largest Muslim ethnoreligious group in South Asia and the second largest in the world after the Arabs. There is also a minority of non-Bengali Muslims. The vast majority of Bangladeshi Muslims are Sunni, followed by minorities of Shia and Ahmadiya. About four percent are non-denominational Muslims. Bangladesh has the fourth-largest Muslim population in the world, and is the third-largest Muslim-majority country (after Indonesia and Pakistan). Sufism has an extensive heritage in the region. Liberal Bengali Islam sometimes clashes with orthodox movements. The largest gathering of Muslims in Bangladesh is the apolitical Bishwa Ijtema, held annually by the orthodox Tablighi Jamaat. The Ijtema is the second-largest Muslim congregation in the world, after the Hajj. The Islamic Foundation is an autonomous government agency responsible for some religious matters under state guidance, including monitoring of sighting of the moon in accordance with the lunar Islamic calendar in order to set festival dates; as well as the charitable tradition of ''zakat''. Public holidays include the Islamic observances of Eid-ul-Fitr, Eid-al-Adha, the Prophet's Birthday, Ashura and Shab-e-Barat.