Laos has three distinct seasons, all with their advantages and disadvantages. The best time to visit overall is the cool season, between November and February. By November, the rains are usually over, all rivers are navigable and roads are pasable.
The hot season, from March to May is very dry, but in some places, around Lao New Year, 14th – 16th April, accomodation of your choice may be harder to find. During this time, some rivers are harder to navigate, and boat journeys in remote places may not be possible.
The rainy season, from June to October, whilst the country at its most beautiful and lush, can be difficult to get around in remote places by road. Rivers are always accesible at this time.