Laos has three distinct seasons, all shaped by the monsoon cycle.
Cool and dry season (November to February) is the best time to visit. Temperatures in the lowlands hover around 20-28°C, humidity is low, and the skies are clear. This is peak tourist season, particularly around Luang Prabang. In the mountains, nights can drop close to freezing -- bring warm layers if you are heading to Phonsavan or the northern highlands.
Hot season (March to May) brings temperatures of 35°C and above in the lowlands, with hazy skies from agricultural burning. It can be oppressively hot, especially in Vientiane and the Mekong valley. The upside is fewer tourists and lower prices.
Rainy season (June to October) sees daily downpours, usually heavy but short. The countryside turns brilliantly green, waterfalls are at their most spectacular, and the Mekong swells dramatically. Travel can be slower -- some rural roads become impassable -- but it is a beautiful time to be in Laos. The rain tends to be worst in August and September.
If you plan to take the slow boat down the Mekong, the dry season is more comfortable, though the river is navigable year-round. For trekking in the north, November to February offers the best conditions.