Hajj is the annual pilgrimage made by Muslims to the holy city of Mecca in Saudi Arabia, in the Middle East.
It takes place during Dhu al-Hijjah, which is the final month of the Islamic calendar.
This year, millions of Muslims from across the world are making the journey to Mecca from 9 to 14 August.
Read on to find out more about why this pilgrimage is so important in the Islamic faith.
Hajj is one of the five pillars of Islam. These are the five key acts which every Muslim is expected to do in their lifetime.
In addition to Hajj, the other important acts are:
- Shahadah - A declaration of faith that every Muslim has to make
- Salat - Muslims pray at specific times five times a day
- Zakat - those within the faith give away a portion of their income to help those in need
- Sawm - Muslims fast for a month during Ramadan
Muslims are required to make the journey to Mecca at least once in their life if they are physically able and can afford to do so.
Mecca is the place where the Islamic religion started. It is significant for two main reasons.
Firstly, it is where the prophet Muhammad was born.
Secondly, it is where he received the first revelations from Allah (Allah is the Arabic word for God) that went on to become the Koran - the holy book read by Muslims.
All Muslims pray in the direction of a sacred building called the Ka'bah, which is found within the Great Mosque of Mecca.
This is the holiest site in Islam and symbolises the oneness of God.
Muslims carry out a number of important rituals while they are on the pilgrimage.
1. Upon arrival at Mecca, they enter a sacred state of purity known as Ihram. Men are required to wear two sheets of white cloth, which are worn in a specific way. Women wear traditional clothing and must cover their head, but not their face. These clothes symbolise the equality of all Muslims before Allah.
2. During the first stage of Hajj, Muslims walk around the Ka'bah in an anti-clockwise direction seven times. This is known as Tawaf and is done to show that all Muslims are equal.
3. The next ritual requires Muslims to run between two hills, Safa and Marwah, seven times. Muslims believe that the prophet Ibrahim's wife Hagar did this when she was in search of water for her infant son Ismail. Ismail is believed to have struck his foot on the ground and this produced a spring of water known as Zamzam. It is common for those on the pilgrimage to take water from Zamzam with them when they go back home.
4. After this, pilgrims travel to the plain of Arafat. This is where the Prophet Muhammad gave his final sermon and Muslims pray to God here for forgiveness and guidance.
5. Next, Muslims stop at three pillars called Jamarat. This is where pilgrims throw stones at three pillars which stand at the place where Satan is believed to have tempted the prophet Abraham.
6. Muslims celebrate the festival Eid ul-Adha at the end of Hajj. This is the second holy festival in the year.
Eid ul-Adha marks Allah appearing to the prophet Ibrahim in a dream and asking him to sacrifice his son Ismail as an act of obedience to God. The Koran then says the devil tempted Ibrahim, by encouraging him to disobey Allah and spare his son.
As Ibrahim was about to kill his son, Allah stopped him and gave him a lamb to sacrifice instead.