The Hajj is a pilgrimage to Mecca undertaken by Moslems from all over the world. It is the "Fifth Pillar of Islam" and there is an obligation on every able bodied Moslem who can afford to go, to undertake this pilgrimage at least once in a lifetime.
In 2008 over 3 million pilgrims visited.
The date is set by the (lunar) Islamic calendar.
Ministry of Foreign Affairs for the Kingdom of Saudi Arabia http://www.mofa.gov.sa/