Low season in Luang Prabang is the period from May to September. I don’t really get why fewer people visit during this time. This is the hot, rainy season but calling it the rainy season can be a little misleading.
The Boat Racing Festival, happens during low season, in September. The festival takes place over a weekend, where villages from around the region compete in dragon boat races on the Nam Khan river. The atmosphere is electric. You can watch the teams doing their evening training in the weeks leading up to the event, and also catch the smaller boat races on the Mekong the week prior to the main event.
Moreover, beautiful Tad Sae Waterfall dries up in the high season but during the rainy season from July and into October, the falls are spectacular and a wonderful place to go and cool off with a refreshing swim.