The world's largest mass gathering is set to begin next week when often more than one million Muslims make the Hajj pilgrimage to Mecca, the holiest city in Islam. This year, the Hajj is expected to begin Monday and end Sept. 26. Here's what that religious journey looks like, in pictures. 

Hajj takes place during the last month of the Islamic year, which every year shifts approximately 11 days earlier against the Gregorian calendar. It involves a number of rituals, one of which includes circling the Kaaba -- a black, cube-like structure also known as the House of God -- the holiest site in Islam. It is located within the Grand Mosque, and Muslims believe Abraham built the Kaaba 4,000 years ago. During Hajj, Muslims are supposed to carry out tawaf, which requires them to circle the Kaaba seven times.

RTR495NL Muslim pilgrims pray around the holy Kaaba during their final circling at the Grand Mosque during the annual hajj pilgrimage in Mecca, Oct. 6, 2014. Photo: Reuters/Muhammad Hamed

Able-bodied Muslims are supposed to carry out the Hajj at least once in their lifetime, as part of one of the five pillars of Islam. The global Muslim population was about 1.6 billion in 2013, according to the Pew Research Center. Nearly one billion live in the Asia-Pacific region, nearly 250 million in sub-Saharan Africa, and 317 million in the Middle East and North Africa. Nearly 2.5 million live in North America. That's a lot of traveling, when it comes to making the pilgrimage to Mecca.

GettyImages-456338594 Muslim pilgrims arrive for the afternoon prayer at Mecca's Grand Mosque, home of the cube-shaped Kaaba or 'House of God' that Muslims believe was built by Abraham 4,000 years ago, Sept. 29, 2014. Photo: Mohammed Al-Shaikh/AFP/Getty Images

Muslims also pray during Hajj.

RTR47Z3V Muslim pilgrims pray at the Grand Mosque during the annual hajj pilgrimage in Mecca Sept. 27, 2014. Photo: Reuters/Muhammad Hamed

Kissing the Kaaba during tawaf is another tradition.

RTX14EK6 Muslim pilgrims touch the holy Kaaba at the Grand Mosque during the annual Hajj pilgrimage to the holy city of Mecca, Oct. 17, 2013. Photo: Reuters/Ibraheem Abu Mustafa

The second day of the pilgrimage is known as the Day of Arafat. On that day, Muslims make their way to Mount Arafat, where they believe the Prophet Muhammad, the founder of Islam, gave his Farewell Sermon during his last year of life. There, Muslims pray from sunup to sundown.

RTR48VER Muslim pilgrims walk while they are sprayed by water as they leave the plains of Arafat during the annual Hajj pilgrimage, outside the holy city of Mecca Oct. 3, 2014. Photo: Reuters/Muhammad Hamed

In preparation for the Hajj, Saudi security forces participate in a military parade. 

GettyImages-456292440 Members of the Saudi special police unit perform during a parade in Mecca, Sept. 28, 2014, as more than one million Muslims arrived in the holy city in the lead up to the annual hajj pilgrimage, one of the world's largest human gatherings. Photo: Mohammed Al-Shaikh/AFP/Getty Images

Because Hajj is the largest gathering of people in the world, preventing the spread of disease is always a concern. In 2014, protecting against the Ebola virus and MERS coronavirus was a primary worry.

GettyImages-456243752 Pilgrims from Asia wear nose and mouth masks on a street of the holy city of Mecca in Saudi Arabia, on Sept. 24, 2014. Photo: AFP/Getty Images

Hajj may be a ritual more than 1,000 years old, but in the 21st century it is infused with modern technology and trends, like selfies, to the chagrin of some clerics.

GettyImages-456666268 Muslim pilgrims pose for a selfie during the 'Jamarat' ritual, the stoning of Satan, in Mina near the holy city of Mecca, on Oct. 4, 2014. Photo: Mohammed Al-Shaikh/AFP/Getty Images

On the third day of the Hajj, Muslims pilgrims throw pea-sized pebbles at pillars that symbolize the devil. They then typically cut their hair or shave their heads. GettyImages-456625334 Muslim pilgrims raise their hair after throwing pebbles at pillars during the "Jamarat" ritual, the stoning of Satan, in Mina near the holy city of Mecca, Oct. 4, 2014. Photo: Mohammed Al-ShaikhAFP/Getty Images