The festival is celebrated over the span of three days and commemorates the end of the rainy season, as well as the change in the flow of the Tonlé Sap River. There are many places where boat racing festivals are held, but the biggest gathering is the festival held in the capital Phnom Penh on the Tonlé Sap River right in front of the Royal Palace of Cambodia.
The boat racing festival has been recorded for a long time in the history of Cambodia as well as some neighboring countries. So far, there are 3 theories about the origin of this festival, with some significant differences: The festival has been celebrated in Cambodia since at least the reign of Jayavarman VII in 1181 AD.; The festival appeared in the Longvek period in 1528 AD; The festival appeared at the same time that Buddhism was introduced to this land.
The festival’s celebrations occur over the span of three days, with the Royal Boat Race on the first day. After the boat race, large lanterns are released as part of the “Bondet Bratib” ceremony at 6:00 pm as representatives from national institutions pray for peace from Preah Mae Kongkea or the Goddess Ganga. Each ministry has its lantern adorned with colorful lights and sets off fireworks to celebrate the river’s rich glory.
The second day of the festival is the day of Og Ambok and involves the worship of the Moon. It takes place at twelve minutes past midnight and consists of a lantern lighting ceremony with prayers to Preah Purthisat, who invented the legend of the moon rabbit. The Og Ambok ceremony involves playing a group game where members must make each other laugh; whoever lasts the longest wins and decides who the loser is. The loser must then eat Ambok with bananas until the end of the day.
On the last day, a ribbon is cut, signifying the end of the boat race and of the Water and Moon Festival.
According to Wikipedia.