More than 1.5 million Muslims from around the world gathered in Mecca, Saudi Arabia, for the Hajj, an Islamic pilgrimage to Islam’s holiest city. The religious trip is a once-in-a-lifetime duty for every Muslim who is financially and physically capable.

Millions of Muslims have gathered in Mecca, marking the beginning of the five-day Hajj pilgrimage—the world's largest annual religious gathering. It is year 1446 in the Islamic calendar. Pilgrims are expected to retrace the steps of the Prophet Muhammad,

Here is a look at the hajj, one of the largest mass gatherings in the world, which starts in Saudi Arabia on Wednesday.