Each year on this day, hundreds of Cambodians head to the Chhoeung Ek killing field site on the outskirts of Phnom Penh. There a ceremony is held and a play put on to recall the victims of the Khmer Rouge.
In Cambodia, May 20 is known as the Day of Anger.
It is a time to remember the millions who died under Pol Pot's Khmer Rouge regime that ruled the country between 1975 and 1979.
More than 1,000 people gathered this year at the Chhoeung Ek memorial site outside Phnom Penh. There were hundreds of Buddhist monks, dignitaries, and many elderly people.
The key part of the ceremony is a macabre play staged by students. About a dozen are cast as black-clad Khmer Rouge soldiers; three dozen more are their victims.
With barked orders, gun shots, screams and tears, the Khmer Rouge and their victims act out the murders, just yards from the mass graves that form this notorious killing field where thousands died.
Pa Socheatvong is a deputy governor of Phnom Penh. He says May 20th was chosen because that was the day in 1976 when the Khmer Rouge instituted their policy to kill people.
"Cambodian people respect the people who were killed in the Pol Pot regime," said Pa Socheatvong. "Pol Pot betrayed the country by using the people's blood as capital, so people [are] very angry with the Pol Pot regime."
The performance by the students is chilling, realistic and disturbing. Many in the crowd wipe away tears during the 15-minute performance.
At the end of the play, other soldiers burst in with guns and Cambodian flags and drive away the Khmer Rouge.
That moment refers to the invasion in late 1978 by Vietnamese troops and Cambodians that saw the end of the Khmer Rouge rule of the country.
The man who sent so many people to their deaths here at the Chhoeung Ek killing field went on trial last year for crimes against humanity and war crimes. His name is Comrade Duch, and he admits he ran the S-21 torture and execution center in Phnom Penh. Judgment in his case is likely to be handed down in the coming weeks.
If Duch is found guilty - as is thought likely - he will be the first former Khmer Rouge to be sentenced for crimes committed at that time. It is a landmark case.
No-one knows how many people died during the Khmer Rouge's rule of Cambodia, estimates range from 1.5 million to three million.