Hajj

Hajj is the fifth and final pillar of faith. It is a pilgrimage to the holy city of Mecca that every Muslim is expected to make at least once in their lifetime.
First, the pilgrims walk counter-clockwise seven times around the Ka'bah, either kissing or touching the stone. Next, they run seven times between the hills of As-Safa and Al-Marwah. This action is symbolic of Hagar's search for water and the miracle of the well, Zamzam. After spending the night at the village of Mina, pilgrims take the next step, the wakuf, together. They meet at the plain of Arafat, about six miles from Mecca. From noon to sunset, they pray quietly. Next, they climb a small mountain called the Mount of Mercy, and they ask God's forgiveness for their sins. They spend the night at Muzdalifah.
Afterwards, they return to the village of Mina where the ritual of "stoning the devil" takes place. Followers gather pebbles which they throw at three pillars. This is symbolic of Abraham throwing stones at the Devil who had disturbed his prayers. On the tenth day, animal sacrifices - sheep or goat - are carried out. Finally, the pilgrimage again goes seven times around the Ka'bah. (Credit to http://www.waupun.k12.wi.us/Policy/other/dickhut/religions/31%20Five%20Pillars%20of%20Islam.html for those two paragraphs)
After the pilgrimage is complete, a 4 day festival is conducted.

People gathered in Mecca