The hajj, one of the largest annual human gatherings in the world, begins on Wednesday in Mecca, Saudi Arabia. Amid rising temperatures and logistical challenges, the pilgrimage has increasingly become a test of endurance both for pilgrims and the Saudi government.

Millions of Muslims from around the world travel to the city to take part; Saudi Arabia said 1,475,230 pilgrims from abroad have arrived since Sunday. Last year, the Saudi government said more than 1,300 pilgrims died, many from Egypt. Most of those who perished had been unregistered, Saudi officials said, meaning they had made the trip without the permits that gave them access to heat protections.

Here’s what to know about this year’s hajj and efforts to make it safer.

What is the hajj?

The hajj is one of Islam’s five pillars, the core practices that define a Muslim’s faith, alongside daily prayer, charity, fasting during the month of Ramadan and the declaration of faith in one God and the Prophet Muhammad as a messenger of God.

For the annual pilgrimage to Mecca, the holiest city in Islam, every able-bodied Muslim who can afford it is expected to undertake the journey at least once.

The hajj takes place once a year during a specific period in the final month of the Islamic calendar. Most pilgrims travel to Saudi Arabia well in advance, using the time to visit the holy city of Medina, pray in Mecca’s grand mosque and perform a lesser pilgrimage called Umrah.