Hajj is the oldest and most sacred ritual of Islam. The pilgrimage focuses on the holy city of Mecca, the birthplace of the Prophet Muhammad. It is the largest pilgrimage in the world and is to be performed at least once by every able bodied Muslim who can afford it. Saudi officials say more than 2.5 million Muslim pilgrims are taking part in the annual Hajj rites, which peak on Saturday. The white robes symbolize purity and equality under God.
Some other interesting facts about the Muslim Hajj pilgrimage here.