The dry season in the kingdom of Cambodia lasts from the beginning of October to the end of April. This period corresponds to a nice climate in South Vietnam, which knows a dry season from November to May, a nice climate in North Vietnam (at the exception of the cold months of December, January and February in the Red River Delta in Northern Vietnam and the mountainous areas by the Chinese border) and in Laos, which also knows a dry season from October to April (with colder months in Northern Laos from late November to late February). It is then convenient to travel during this period if you plan on traveling throughout the whole country, or even throughout the whole Southeast Asian region, organizing a multi-country trip including the best of Laos, Vietnam and Cambodia.
If you are willing to visit Cambodia at another time of the year, during the period from May to September, the temperatures are considerably higher, rising during the first few months and so do the rainfalls. The good thing about it is that the first two months of the rain season know relatively few rainfalls and you can often enjoy mostly warm, sunny days, while the rain will start in the late afternoon and during the night. The humidity rate rises with the rainfalls and while the temperatures are the warmest at the end of the dry season and beginning of the rain season (in March, April, May and June, the temperatures can reach 35 degrees and above at some points), the weather remains warm during the whole rainy season, making it difficult to travel to Cambodia during the months of June, July, August, September and October.
If you are on a budget, however, the first and last months of the rain season are the perfect times to visit the country and benefit from exceptional services, hotels, excursions at a much lower price than during the high season. Also, choose to travel during these periods and you will meet far less tourists during the activities and excursions, as well as in the hotels, which is the perfect time to ask for a better rate and a free upgrade!
Also, Cambodia has interesting events and festivals you might want to add to your agenda if your exotic holidays are not all about the weather. The most renowned festival (and most important holidays) is the Khmer New Year. During three days in April, Cambodian population celebrates the Bon Chol Chhnam Thmei as most cities completely shut down for a week, which is quite similar to the Vietnamese Lunar New Year or Tet. This period corresponds to the Songkran in Laos, Thailand and Myanmar, but Khmer people celebrate it in a way that also reminds Vietnamese way of celebrating the Tet, by going back to their family hometown, spending time with their families, partying and drinking, as well as visiting the pagodas.
Other festivals count the water festival or “Bon Om Touk”, that takes place in November to celebrate the change in flowing direction of the TonLe Sap river's waters, the Pchum Ben or ancestor's day that lasts 2 weeks from mid September and during which Cambodian people offer goods to monks (and indirectly to their ancestors).