It celebrates the end of the rainy season, the start of the fishing season, and also the unique natural phenomenon the reversal of the current on the Tonle Sap. 

During the dry season the waters  back up into Tonle Sap Lake, they would now begin to empty into the Mekong.  

Had I know about it in advance I might have been tempted to go to Phnom Penh where the festival rivals Mardi Gras.

Women were preparing and cooking food on the banks of the Stung Siem Reap River.  

Vendors were preparing to hawk every conceivable  essential for a good time.

Men were playing cards (sic) and creating elaborately decorated platforms on dugout canoes and other small boats..  Later they added lights.  

Do they also serve who only stand and watch?

The event turned out to be the biggest dragon boat festival of the year.  We watched them practice in the morning and returned after lunch to once  more join hundreds of people in holiday mood on the banks of a river.