The dry season runs from February to August, with the best conditions in March, April, and May — warm but not brutally hot, and the rivers running at manageable levels for cave tours. June and July are hotter and more crowded. August can still be good but marks the beginning of the monsoon transition.
The wet season from September to January brings heavy rain and the risk of flooding in the cave entrances and river valleys. Some cave tours are cancelled or curtailed during this period; the Phong Nha boat tour in particular depends on river levels staying below certain thresholds. Flooding can close the park entirely for short periods. Check conditions with operators before travelling between September and January.
The park is busiest during Vietnamese public holidays (April 30, September 2) and the summer school break (June–August), when domestic tourism surges. If you can, visit in March or April for good weather and smaller crowds.