What is the Hajj pilgrimage and what is Eid al-Adha? Why are they significant for Muslims?

Once a year, large numbers of Muslim pilgrims converging on Saudi Arabia unite in religious rituals and acts of worship as they perform the Hajj, one of the pillars of Islam. They fulfill a religious obligation, immersing themselves in what can be a spiritual experience of a lifetime for them and a chance to seek God's forgiveness and the erasure of past sins.

Here's a look at the pilgrimage and its significance to Muslims.

WHAT IS THE HAJJ?

The Hajj is the annual Islamic pilgrimage to Mecca in Saudi Arabia and involves a series of religious rituals. It's required once in a lifetime of every Muslim who can afford it and is physically able to make it. Some Muslims make the journey more than once.

It is also one of the Five Pillars of Islam, in addition to the profession of faith, prayer, almsgiving and fasting.

WHEN IS THE HAJJ?

The Hajj occurs once a year during the lunar month of Dhul-Hijja, the 12th and final month of the Islamic calendar year. This year, the Hajj will start on June 4.

When the Hajj falls during the summer months, the intense heat can be especially challenging. Amid extreme high temperatures last year, more than 1,300 people died during the Hajj, Saudi authorities announced then. The country's health minister said at the time that the vast majority of the fatalities were unauthorized pilgrims who walked long distances under the sun.

WHAT'S THE SIGNIFICANCE OF THE HAJJ TO MUSLIMS?

For pilgrims, performing the Hajj fulfills a religious obligation and is also a deeply spiritual experience. It's seen as a chance to seek God's forgiveness for past sins, to grow closer to God and to walk in the footsteps of prophets.

Communally, the Hajj unites Muslims of diverse races, ethnicities, languages and economic classes from around the world in performing religious rituals and acts of worshipping God at the same time and place. This leaves many feeling a sense of unity, connection, humility and equality. Pilgrims also show up with their own personal appeals, wishes and experiences.

Many pilgrims bring with them prayer requests from family and friends that they would like to be said on their behalf.

Some spend years hoping and praying to one day perform the Hajj or saving up money and waiting for a permit to embark on the trip.

Ahead of the journey, preparations may include packing various essentials for the physically demanding trip, seeking tips from those who've performed the pilgrimage before, attending lectures or consulting other educational material on how to properly perform a series of Hajj rituals as well as spiritually readying oneself.

WHAT ARE SOME OF THE RITUALS THAT PILGRIMS PERFORM?

Pilgrims make the intention to perform the Hajj and they enter a state of ''ihram.'' Being in ihram includes abiding by certain rules and prohibitions. For instance, men are not to wear regular sewn or stitched clothes that encircle the body, such as shirts, during ihram. Instead, there are simple ihram cloth garments for men; scholars say the purpose is to discard luxuries and vanity, shed worldly status symbols and immerse the pilgrim in humility and devotion to God.

A spiritual highlight of the Hajj for many is standing on the plain of Arafat, where pilgrims praise God, plead for forgiveness and make supplications. Many raise their hands in worship with tears streaming down their faces.

Other rituals include performing ''tawaf,'' circumambulating, or circling the Kaaba in Mecca counterclockwise seven times. The Kaaba, a cube-shaped structure, is the focal point toward which Muslims face during their daily prayers from anywhere in the world.

Among other rituals, pilgrims also retrace the path of Hagar, or Hajar, the wife of Prophet Ibrahim, Abraham to Jews and Christians, who Muslims believe ran between two hills seven times searching for water for her son.

WHAT IS EID AL-ADHA?

Eid al-Adha, or the ''Feast of Sacrifice,'' is the Islamic holiday that begins on the 10th day of the Islamic lunar month of Dhul-Hijja, during the Hajj.

A joyous occasion celebrated by Muslims around the world, Eid al-Adha marks Ibrahim's test of faith and his willingness to sacrifice his son as an act of submission to God. During the festive holiday, Muslims slaughter sheep or cattle and distribute some meat to poor people.

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