Exactly 30 years ago, a coalition of Cambodian and Vietnamese troops forced Pol Pot and his followers from power - after a four-year reign which left as many as two million people dead.
In the end, the Khmer Rouge were quietly toppled
But, as the BBC's Guy De Launey reports, Victory Day is not universally celebrated in Cambodia.
Even in 1979 it was a strange kind of victory.
There was no hero's welcome for the conquering troops. But nor was there a nervous population, fearful about the intentions of the incoming army. Because Phnom Penh was almost completely deserted.
The Khmer Rouge had forced millions of city-dwellers to march to the countryside when it took power in April 1975. In the intervening years only a small number of party workers were allowed to live in the capital, and they had fled as the Vietnamese-backed forces approached.
Vann Nath was one of the few Phnom Penh residents to witness the final hours of Pol Pot's murderous government. As an inmate of the notorious Tuol Sleng prison, however, his was not the most comfortable vantage point.
"When the Vietnamese arrived there was a group of us locked in one room in Tuol Sleng. I heard some gunshots, outside on the main street," he remembers.
Soon after, with no little disbelief, he walked free. "In all there were seven of us who survived Tuol Sleng. To this day I still don't know why I was allowed to live."
Vann Nath had beaten formidable odds. Around 15,000 prisoners had passed through Tuol Sleng, but only a handful survived. Many died during the prison's systematic torture process; the rest were bludgeoned to death in the so-called killing fields on the outskirts of Phnom Penh.
Two million dead
Under the Khmer Rouge, tens of thousands shared a similar fate, executed as "enemies of the revolution", often on the flimsiest of pretexts. Others perished on the gruelling journey from the cities to rural villages, or starved to death while working as forced labourers on collective farms.
As many as two million Cambodians are thought to have died because of the policies of Pol Pot's government and the actions of Khmer Rouge members.
The current government says the end of that era is a cause for celebration. Prime Minister Hun Sen was among the Cambodian troops who joined Vietnamese forces to oust the Khmer Rouge.
Skulls litter the Khmer Rouge's "killing fields"
In the run-up to the anniversary, the governing Cambodian People's Party has called 7 January a "historical day" and a "new birthday" for the country.
The CPP is planning to mark the 30th anniversary in grand style. To ensure maximum attendance at a rally in Phnom Penh's Olympic Stadium, thousands of children from nearby schools will each receive cash for food and drink, as well as T-shirts and caps in CPP colours.
The party political nature of the celebrations is an indication that not all Cambodians acknowledge 7 January as a day of liberation. Opposition leaders sometimes refer to it as a Vietnamese invasion.
Pen Sovann feels that more deeply than most. He was one of the founders of the anti-Khmer Rouge front, and became prime minister in 1981 before a spectacular fall from grace.
Vietnam has had a long influence on Cambodian affairs
Disagreements with the government's Vietnamese sponsors earned him first the sack, then a 10-year spell in a Hanoi prison.
"It was liberation at first. They couldn't stand and watch their neighbour's house burn, so they provided the water to help put out the fire," he says now.
"But I wasn't happy that they were trying to exert their influence over Cambodia. They were getting stronger and stronger, and even now they have more influence than the current Cambodian leaders."
Others, however, are happy to embrace 7 January as a day of liberation from the Khmer Rouge, whatever they feel about the decade-long presence of Vietnamese troops in Cambodia and the on-going close relations between the two countries.
It can hardly compare to the years of madness which had seen Khmer turn against Khmer.
For many, more than anything else, the anniversary provides an opportunity to keep alive the memory of what happened in the Pol Pot era.
Pum Chantinie is now the secretary general of the Cambodian Red Cross. Along with the surviving members of her family, she walked hundreds of kilometres from the countryside back to Phnom Penh in January 1979, pushing a hand-made cart - only to discover their old house wrecked and surrounded by barbed wire.
"Our family gathers and we tell stories about our lives during the Khmer Rouge regime to our young generation," she says. "They ask me, 'Why, why why?' and I tell them I don't know why they did this - to our family, to Cambodian people."
Most Cambodians are too young to remember the Pol Pot era, or even the years of civil war which followed. But the anniversary should be followed, within months, by another reminder of what happened. The first trial at the Khmer Rouge Tribunal may start as early as March.
In the dock, charged with crimes against humanity, will be Vann Nath's former jailer, the man best known as Comrade Duch. All the indications are that he is willing to reveal what he knows about how the Khmer Rouge leadership made the decisions which led to so many deaths.
That may not be a cause for celebration. But it may at least answer some of the questions that people like Pum Chantinie have struggled with for so many years.