November through April is the dry season and the best time for the Loop. Roads are passable, caves can be entered safely, and the nights are cool. February and March offer the best balance of comfortable temperatures and low crowds. The dry season runs later here than in northern Laos, and April can be very hot on the valley roads.
The rainy season (May–October) makes the Loop more adventurous: some roads become very muddy, river crossings rise, and the cave sections can be impassable if flooding raises the internal water levels. Kong Lor Cave closes periodically during high water; check conditions in Thakhek before setting out in the wet season. The upside is that the landscape is dramatically green and the karst formations look their best in the rain.
The caves near town along Route 12 are accessible year-round, and Thakhek itself — the fountain square, the riverfront — is pleasant in any season.