'The Hajj in Makkah' photo (c) 2008, menj - license: http://creativecommons.org/licenses/by-nd/2.0/

Millions of Muslims make the annual pilgrimage, called the Hajj, to Mecca in Saudi Arabia. Because the Islamic calendar is a lunar calendar based on the motion of the moon, the time of the Hajj is different time each year in terms of the Gregorian calendar.

In 2013, it takes place between October 13-18.

You might be interested in The Best Sites To Learn And Teach About The Hajj.