Millions of Muslim pilgrims, all dressed in white, assembled from early Thursday to Mount Arafat in Saudi Arabia's west to take part in the major worship of the annual Muslim Pilgrimage known as Hajj. Many pilgrims had camped overnight in the sprawling plain surrounding Mount Arafat but the majority began arriving at dawn. Men, women, and children from 189 countries streamed to the site, some setting up small tents in which they slept and prayed. After dawn prayers, pilgrims headed to the small hill in Arafat plain named the "Mount of Mercy" as others made themselves comfortable between its huge rocks. Many prayed, tears streaming down their faces. After sunset, the pilgrims will head to Muzdalifah, between Mina and Arafat, where they collect stones to throw at the devil, one of the last worships which takes place Friday and marks the first day of Eid al-Adha [the feast of sacrifice]. The symbolic "stoning of the devil" is followed by the religious sacrifice of an animal, usually a lamb. During the remaining three days of the Hajj, the pilgrims continue the stoning worship before performing the circumambulation of the Kaaba shrine in Mecca and heading home. The Hajj is one of the "five pillars" of Islam that every capable Muslim must perform at least once, according to the Islamic religious instructions. Saudi Arabia spent billions of US dollars over the last ten years in order to improve the roads and facilities used by the pilgrims during the Hajj, particularly on enlarging the Holy Mosque of Mecca which contains the Kaaba shrine, and expanding the stoning area where hundreds of pilgrims died in the past due to jostle among the crowd.