Each year between 2 to 3 million Muslims from around the world take part in a five-day pilgrimage in Mecca called the hajj. They circle Islam's most sacred site, the cube-shaped Kaaba, and take part in a series of intricate rituals. Here's a look at some questions and answers about Islam's holiest site and the pilgrimage:

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WHAT IS THE PURPOSE OF THE HAJJ?

Muslims believe that taking part in the hajj pilgrimage leads to a spiritual rebirth. The Quran holds that on the Day of Judgment, God will weigh a person's sins and good deeds and based on that they will face heaven or hell. The hajj is seen as a chance to wipe clean past sins and start fresh. Many unveiled women return from the hajj covering their hair in an effort to remain devout.

The hajj is a main pillar of Islam, required of all able-bodied Muslims to perform once in their lifetime.

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WHAT IS THE HISTORY OF THE HAJJ?

While following a route the Prophet Muhammad once walked, the rites of hajj are believed to ultimately trace the footsteps of the prophets Ibrahim and Ismail, or Abraham and Ishmael as they are named in the Bible. Pilgrims also trace the path of Hagar, the wife of Ibrahim, who Muslims believe ran between two hills seven times searching for water for her dying son. Tradition holds that God then brought forth a spring that runs to this day. That spring, known as the sacred well of Zamzam, is believed to possess healing powers and pilgrims often return from the hajj with bottles of its water as gifts.

Muslims believe Ibrahim's faith was tested when faced with a command from God to kill his only son Ismail. Upon hearing the command, Ibrahim was prepared to submit to God's will. He then received a revelation that his sacrifice had been fulfilled by his willingness to submit, and his son was spared. Christians and Jews believe a similar version, but believe Abraham's other son Isaac was the intended sacrifice.

The final days of hajj coincide with Eid al-Adha, or festival of sacrifice, celebrated by Muslims around the world to commemorate Ibrahim's test-of-faith. During the three-day Eid, Muslims slaughter livestock and distribute the meat to the poor.

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WHAT ARE THE RITUALS PERFORMED DURING THE HAJJ?

The hajj includes a number of physically demanding rites. It traditionally begins in Mecca, Saudi Arabia with a smaller pilgrimage called the "umrah", which can be performed year-round. Pilgrims enter into a state of physical and spiritual purity known as "ihram."

Women forgo makeup and perfume and wear loose-fitting clothing and a head covering, while men are dressed in seamless, white terrycloth garments.

The umrah begins with circling the Kaaba counter-clockwise seven times while reciting supplications to God. Pilgrims then walk between the two hills traveled by Hagar. Both rites are performed in Mecca's Grand Mosque.

On the eighth day of the Islamic month of Dhul Hijja, Muslims in ihram traditionally head five kilometers (three miles) from the city of Mecca to Mina, a massive valley that houses more than 160,000 tents where pilgrims spend the night.

The next morning, pilgrims head to Mount Arafat, about 20 kilometers (12 miles) east of Mecca. This day marks the pinnacle of hajj. The Prophet Muhammad is believed to have said that hajj is Arafat, in reference to the day spent there and its importance. Pilgrims are packed shoulder to shoulder there, with some men and women openly weeping and praying.

Tens of thousands scale a hill called Jabal al-Rahma, or mountain of mercy, in Arafat. It is here where Muhammad delivered his final sermon during the hajj.

Around sunset, pilgrims head to an area called Muzdalifa, nine kilometers (5.5 miles) west of Arafat. Many walk the distance by foot, others use buses. They spend the night there and pick up pebbles along the way that will be used in a symbolic stoning of the devil back in Mina. It was in Mina where Muslims believe the devil tried to talk Ibrahim out of submitting to God's will and sacrificing his son.

The last three days of the hajj are marked by three events: a final circling of the Kaaba, casting stones in Mina and removing the ihram. Men often shave their heads at the end in a sign of renewal.

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HOW DO THE DISABLED AND ELDERLY PERFORM THE HAJJ?

Many elderly and disabled people, who have waited a lifetime to perform the hajj, travel with relatives who help them along the way. They use wheelchairs and are assisted by ramps in the Grand Mosque to avoid the massive crowds. Despite the physical challenges of the hajj, many people rely on canes or crutches and insist on walking the routes to trace the footsteps of prophets before them.

Those who cannot afford the hajj are sometimes financed by charities or community leaders. Others save their entire lives to make the journey. A few even walk thousands of miles by foot to Saudi Arabia, taking months to arrive.

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WHAT ARE THE MOST IMPORTANT LESSONS OF THE HAJJ?

The state of ihram for men and women is aimed at shedding symbols of materialism, giving up worldly pleasures and focusing on the inner self over outward appearance. Sexual intercourse among spouses is not permitted when one is in a state of ihram, neither is trimming hair or nails. The white ihram garments are forbidden to contain any stitching — a restriction meant to emphasize the equality of all Muslims and prevent wealthier pilgrims from differentiating themselves with more elaborate garments.

It is also forbidden for pilgrims to argue, fight or lose their tempers during the hajj. Inevitably, though, the massive crowds and physical exhaustion of the journey test pilgrims' patience and tolerance.

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WHY IS THE KAABA SO IMPORTANT TO MUSLIMS AND TO THE HAJJ?

Islamic tradition holds that the Kaaba was built by Ibrahim and his son Ismail as a house of monotheistic worship thousands of years ago. Over the years, the Kaaba was reconstructed and drew a wide range of people to it for pilgrimage, including ancient Christian communities that once lived in the Arabian Peninsula. In pre-Islamic times, the Kaaba was also used to house pagan idols worshipped by the local tribes.

Muslims do not worship the Kaaba, however it is Islam's most sacred site because it represents the metaphorical house of God and the oneness of God in Islam. Observant Muslims around the world face toward the Kaaba during the five daily prayers.

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