More than two million Muslims from around the world are expected to gather on Friday at Mount Arafat in Saudi Arabia for a day of prayer during the annual pilgrimage, or Hajj.
Mount Arafat is near the site of the prophet Muhammad's farewell address.
At night, the pilgrims will retreat to a place called "Muzdalifah."
The Hajj began Thursday under heavy security. The pilgrims, in white robes symbolizing purity, traveled from the Saudi city of Mecca to a tent city in the valley of Mina, marking the start of the grueling five-day ritual.
On Saturday, there is a ceremony in Mina symbolizing the stoning of the devil. Last January, at the stoning ceremony on the Jamarat bridge, more than 360 worshippers died in a stampede.
Saudi authorities have spent more than one billion dollars renovating the site to make it safe.
The Hajj is one of the five pillars (obligations) of Islam. Every Muslim who is able to make the trip is required to do Hajj at least once.