I'm in Vientiane Laos and there has been a buildup to a festival for at least a week and the festival finally started officially yesterday with October 20 being the final day.
Wikipedia article on "Boun Song Hua". Other websites about the festival, none of which seem to include the crucial details: laos-guide-999, tourismlaos, vientiane-hotel-link.
Vientiane is situated on the Mekong River but so far I haven't seen a boat anywhere, on the river or otherwise. Just sound stages, kiddie rides, sideshows, makeshift restaurants, and many many many stalls along the main road selling everything from cars and TVs to clothing to street food.
I can't seem to find a program anywhere that lists the actual boat racing. Are there heats or just one race on the last day? Where are the boats? When are the races?