As you can see from the map below, Cambodia has two distinct seasons -- the wet and the dry.
The wet season comes courtesy of the southwest monsoon which blows from May to October. In the peak of the wet, from July to September it can rain as much as two out of every three days. Rainy days tend to have a few hours of heavy rain rather than all-day downpours, though the latter do occur -- you will get wet travelling in a Cambodian wet season. The dry season runs from October to April, when the dusty northeast monsoon arrives. November through January is quite cool (high 20's) while March can be uncomfortable and April excruciatingly hot.
For more detailed information on Cambodia's weather, including detailed climate statistics for Phnom Penh, please see our main Cambodian weather page.
Given Cambodia's recent history, detailed climatic data was a little bit tricky to find so some of the above is based on somewhat scant information. If you happen to be propping open your door with a copy of Cambodia's weather almanac for 2007, we'd love a copy. In the mean time, this is the best we could rustle up.