The FARC rebel group is expected to sign a peace agreement with the government to end half a century of civil war.
The conflict has killed more than 220,000 and displaced millions of people.
It comes as the group meets for the last time as an armed movement and prepares to transform itself into a political one.
The Colombian government and the Revolutionary Armed Forces of Colombia, known as the FARC, signed a bilateral ceasefire and end to hostilities in June.
It was reached after almost four years of talks in Cuba.
The Colombian government negotiator called it the best deal possible.
Kristian Herbolzheimer, from Conciliation Resources, a non-government agency working to end armed conflict around the world, has told Al Jazeera it's a major step.
"Colombia's never been so close to peace as today, and the prospects for a sucessful implementaion of a peace agreement are also very good. So definitely this is a time of watershed in Colombian history."
Bernardo Perez-Salazar, from the Catholic University of Colombia, says the deal is not without its critics.
"Critics are not happy with the fact that this has been the result of a political negotiation. Critics would like to have had the end of the war as a military outcome and so they would have liked for there to have been a capitulation instead of a political negotiation, so that means that most of the responsibility for all the atrocities will not be only on the side of the guerrilla, but will have to be shared by the rest of society - by the army as well as by businessmen that have been involved one way or the other supporting one or the other side - so that means that the peace process will bring costs to all sides."
The deal still needs to be signed and voted on in a referendum in October.
Oxford University's Annette Idler has told Al Jazeera many Colombians aren't confident about the deal.
"Well it's not that clear for the casual observer whether there will be peace or not and the country is really divided here. But of course there is a lot of enthusiam and people are looking forward to it. Some people are concerned about the ex-combatants that they may not be able to reintegrate into civilian life, some others in rural areas - where I've done my field work - they are concerned because they are other armed groups as well so they are not sure that this peace will hold throughout the territory because of the presence of those other groups. But overall we all very much hope that there will be a 'Yes' vote in October."
The historic peace deal demands fighters hand over their weapons, reintegrate into civil society, and participate in politics.
Annette Idler says it's quite possible some fighters won't comply with the terms of the agreement.
"The FARC are a very hierarchical organisation and Timoshenko, the leader, has given the order for the definite ceasefire. And so we hope that most of them will comply, but there is one front - the Front One - that has already declared that they might not join the demobilisation process, so they might join or create their own group. There's also a risk that some of the ex-combatants might simply join criminal groups."
The start of the talks in Cuba more than three years ago was the first step in the peace process in a decade.
An earlier bid by former Colombian president, Andres Pastrana, had collapsed.
In 1998, President Pastrana and then-FARC leader Manuel Marulanda had agreed to the creation of demilitarised zone.
But during the calm the rebels rebuilt their military operations and established a multibillion-dollar drug-trafficking network.
And the violence did not stop.
The talks were ultimately broken off in 2002 when the rebels hijacked a plane and the new government responded by bombing rebel jungle camps.
Ten years later, President Pastrana said the FARC was much weaker as it went into talks in Oslo, in October 2012.
The failure of those talks led to the election of conservative Alvaro Uribe with a clear mandate to act against the FARC.
As the army continued its offensive, the rebels fought back with devastating attacks in Bogota and a series of high-profile kidnappings.
It wasn't until the election of current President Juan Manuel Santos that negotiations resumed.
FARC leader Rodrigo Londono, known as "Timoshenko," sought what he called a "civilised dialogue" to end the bloodshed.
Both leaders announced in September last year that FARC rebels would disarm within 60 days of signing the deal.
Both sides have agreed to the creation of special tribunals to try former combatants, and have embraced an amnesty that would exclude those who committed war crimes or crimes against humanity, and provide reparations for victims.
It followed an earlier partial agreement on cooperating to end the illegal drug trade, and other agreements on land reform and the legal political participation for rebels once they disarm.
Bernardo Perez-Salazar says Colombia has had some success in reintegrating fighters into society.
"The difference this time is that, most likely, most of the reintegration into civil life will not be in urban areas as it was in the past but this time it will probably be in the same rural areas where most of the ex-combatants come from and they'll be probably be organised in cooperatives and they'll probably have productive projects. So I think that the process will be more controlled. Probably you'll have people that will go into criminal activity, but I think most of that will be controlled by the scheme that the FARC are putting in place once they become a political movement and they also want to have an economic base for their ex-combatants, so that looks pretty good."
The FARC grew out of a 1960s Marxist-inspired peasant movement demanding land reform, and has been fighting successive governments ever since.
The conflict has created one of the world's highest internally displaced populations.
