This is a journey of faith to the holiest sites of Islam, in and around Mecca in Saudi Arabia, and only the poor and infirm are exempted from performing this duty.
Consisting of ceremonies and rituals symbolizing the key concepts of Muslim belief, the hajj commemorates the trials of the Prophet Abraham and his family. Lasting two weeks, the gathering in the desert also enables Muslims of all races and backgrounds to express solidarity with each other in their faith.
The experience is spiritually overwhelming, according to Professor Abdel Halim, the director of the Islamic Institute of the School of Oriental and African Studies in London. "People throughout their lifetimes are yearning to do that [pilgrimage] and, once you get there, it really is a very highly charged, peaceful, spiritual experience, from which people emerge different than before they started," he told RFE/RL.
This year, some 2 million pilgrims are expected at the hajj, which will begin in early February, with the exact date being governed by the sighting of the crescent moon by experts.
For the first time, the Saudi Arabian authorities have decided to allow foreign airlines to fly to the holy city of Medina during the hajj, in order to ease the movement of the huge number of pilgrims. Mecca has no airport, and previously only Saudi Arabian Airlines could fly into Medina, with the foreign carriers having to deposit pilgrims in the "secular" cities of Riyadh or Jeddah.
Saudi security forces will be on high alert, given the risk of terrorist incidents. The pro-Western Saudi Arabian authorities have been in recent years the target of attacks by Islamic extremists. There have been a series of bombings and shoot-outs in Saudi cities.
And the hajj season offers opportunities for terrorists, insofar as the number of passengers arriving makes proper security screening impossible. The research director of the Geneva Centre for Security Policy, Shahram Chubin, said the security situation is, in a word, "complicated."
"Obviously there will be a lot of 'soft' spots in terms of targets for terrorists, so I assume it is right to call attention to the combination of the fact that a large number of people will be coming, the security forces may be overstretched, and there is the possibility of somebody trying to use that occasion to undermine the [Saudi] regime in some way," Chubin said.
Chubin says he does not believe terrorists would shrink from acts of violence because of the religious nature of the occasion. He said in general, terrorists in the Muslim world "are not guided by a particularly hard interpretation of the scriptures, of the Koran. They interpret it in terms of their interests, and they say if this is a corrupt regime, it should be got rid of, and we have every right under the Koran to do it."
However, Halim of the Islamic Institute in London takes a different view. He says that no terrorists, if they are Muslims, will come anywhere near the hajj. "No Muslim will take this opportunity because there is an emphasis in the Koran about the safety and peace of Mecca and all the area where the hajj takes place, and I do not believe they would do [any act of terrorism] because it would be a tremendous propaganda blow against themselves," he said.
In order to regulate the numbers of pilgrims, the Saudis have applied a formula under which each Muslim country is allowed to send 0.01 percent of their Muslim population.
This means that Indonesia, as the world's most populous Muslim country, may send 205,000 pilgrims this year.
Indonesia originally asked the Saudis for permission to send more -- another 30,000 people. Assuming this request would be granted, the Indonesian authorities went ahead and issued travel tickets. But much to the annoyance of the would-be pilgrims that bought the tickets, the Saudis turned down the request.
Iraq, however, will be sending a much bigger contingent than last year. A member of the U.S.-backed Iraqi Governing Council, Ibrahim al-Ja'fari, said last month that Saudi Arabia had agreed to more than double the number of Iraqi pilgrims it would allow to perform the hajj. That means some 30,000 Iraqis will be going -- which is still only about one-eighth of the number of Iraqis who applied to attend this year.
In recent years, Iraq's annual participation shrank to some 13,000 pilgrims, reflecting the estrangement between Baghdad and Riyadh since the first Gulf War of 1991.