Saturday is really big day for Muslims, as it is the start of a religious festival called the Hajj. It's one of the five most important things in Islam, called the pillars.
Millions of Muslims are heading to the holy city of Mecca in Saudi Arabia to pray and take part in the Hajj.
Every Muslim is expected to go to Mecca to take part in the Hajj at least once in their life, as long as they are healthy and can afford to get there.
The journey to Mecca is called a pilgrimage. Pilgrims have to carry out a number of tasks during their journey to Mecca.
The final one of the tasks is getting to something called the Kaaba, which is represented by a giant cube in the centre of the Great Mosque in Mecca.