The commemoration ceremony, known as Arbaeen, marks the end of the 40-day mourning period for Imam Hussein, the grandson of the Prophet Mohammed.
The religious ritual draws pilgrims to visit the gold-domed shrine containing Imam Hussein.
This year, the pilgrimage is set to take place on September 16 and 17.
Imam Hussein was killed in battle at Karbala in the 7th century along with his family members and close aides. Those who survived were taken to Damascus, Syria and held captive.
It started when Imam Hussein protested against the second Umayyad caliph, Yazid bin Muawiyah, who was based in Damascus.
His death is considered as a defining moment for Shiite Muslims.
What is Arbaeen?
The Arbaeen pilgrimage is perhaps Iraq's most “symbolically potent event”, Marsin Alshamary, a research fellow at Harvard University's Middle East told The National.
“It attracts both the religious and the non-practising and is an act of community and gathering. Religiously, it is what defines Shiism and, for [Shiite] individuals all over the world, it is a cherished practice,” Ms Al Shammary said.
However, politically, it is a “representation of overcoming oppression and has been utilised as such by savvy politicians over the years”.
What happens during the pilgrimage?
The pilgrimage consists of a long walk from different areas inside and outside of Iraq.
“The whole thing started when people used to make the visit. First of all there was no cars. Those who owned a camel or horse or any other form of an animal would take that and those who [did not] would walk,” said Abbas Kadhim, director of the Atlantic Council's Iraq Initiative.
Walking allows pilgrims to feel the suffering of Imam Hussein's family, who walked from Damascus to Karbala, Mr Kadhim said.
“It is said the family of Imam Hussein, after they were released from captivity, visited the place and they walked there. People say I’m going to share with them that pain and experience,” he told The National.
“In those days, life was dangerous, it was understood that the more hardship you go through, the more rewards you get.”
During the walk, pilgrims are met by volunteers who have set up tents along the roads to distribute free food and drinks and offer places for them to rest.
This shows the generosity of Iraqis, Mr Kadhim said.
Why have there been attempts to ban the pilgrimage?
During the reign of former Iraqi president Saddam Hussein, the pilgrimage was “heavily surveilled by the government”.
“In some years, it even tried to ban it as it brought a lot of trouble to participants,” Ms Al Shammary said.
The US-led invasion of Iraq in 2003 ended Saddam's three-decade rule, during which he persecuted groups such as the Shiites and Kurds.
“After 2003, the first pilgrimage was a mass outpouring for people who had wanted to participate for years but were unable to,” she said.
Additionally, “Iraq was no longer a pariah state so foreigners could also come and participate”, she said, adding that the pilgrim numbers grew every year.
Why do so many people take part?
Arbaeen is known to be the biggest annual gathering of Shiite Muslims worldwide.
The most important development is the freedom that was given to people to exercise their religious rituals and to visit the shrines without any hindrance.
“Most of the history, in the past 1,400 years, Shiites have not been so free as compared to after 2003,” Mr Kadhim told The National.
“Various governments took different policies, some banned any manifestation to the family of the Prophet and, especially, Imam Hussein,” he said.
They faced harsh consequences such as death or prison sentences.
“In 1983 to 1984, Baath [the former ruling political party] archives tell us that close to 500,000 people participated and they faced dire consequences but they did it anyway,” he said.
The freedom that people enjoy now has encouraged Shiites to join the pilgrimage over the years and that is why we see huge numbers of participants, Mr Kadhim said.
The number of visitors has risen since 2014, which has been attributed to the easing of visa requirements for Iranians, especially as they make the largest number of foreigners.
It is known that Sunnis, Christians and other sects and religious minorities usually take part in the pilgrimage.