Large numbers of Muslims converge once a year on Saudi Arabia to unite in a series of religious rituals and acts of worship as they perform the Hajj, the pilgrimage that is one of the pillars of Islam.

More than a million worshippers were set to join Islam’s most important rite under a beating sun as the hajj pilgrimage kicked off Wednesday, with authorities scrambling to avoid last year’s 1,000-plus deaths in sweltering heat.

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