Laos is belonging to tropical and subtropical monsoon climate, specifically, it is tropical in the plains, and subtropical in the mountains and the far north.
The best time to visit, based on the weather, is in the dry season, especially from November to February, when the weather is cool and dry. However, this period is also the peak season, with more travelers. So it may be crowded and the prices may rise. It is advisable to plan ahead and make reservations in advance.
If you'd prefer visiting Laos at a quiet time, the wet season is considerable. Though there is one or two raining hours a day. Then you’ll find everything is greener after the sun comes out.