This is Victory over Genocide Day (Cambodia Victory Day) that marks the end of the genocide that lasted from 1975 till 1979.
Starting from 1975 Pol Pot led the Khmer Rouge regime that was marked by changing the name of Cambodia to Democratic Kampuchea.
The ruling party was responsible in death of over 2 million people who were executed and died of starvation, diseases or overwork.
The regime ended only on January 7, 1979 when Vietnamese troops arrived in Cambodia and started the assault that led to the toppling of Pol Pot and the fall of the regime.
January 7 is the celebration day for Cambodia.