Cambodia's climate is tropical with seasonal monsoons. There are two distinct seasons, the rainy and dry. Temperatures during the rainy season, between June and October, average 81°F to 95°F (27°C to 35°C). The dry season can be divided into cool months, from November till February, with temperatures averaging 63°F to 81°F (17°C to 27°C); and hot months, from March till May, with temperatures averaging 84°F to 100°F (29°C to 38°C). The cooler, dry months of November to February are a pleasant time to visit Cambodia, but the best time to visit does vary depending on desired activities.
The best time to visit Laos is between November and March when it is dry and temperatures are pleasant, although this is high tourist season and accommodation prices may be higher than they are at other times of the year. If you are travelling on a budget it may actually be better to brave the wet season. Having said that, Laos is still an off-the-beaten-track travel destination and you are unlikely to struggle with crowds even during the peak season.