Cambodia can be visited at any time of year. The ideal months are December and January, when humidity levels are relatively low, there is little rainfall and a cooling breeze whips across the land, but this is also peak season when the majority of visitors descend on the country.
From early February temperatures keep rising until the killer month, April, when the mercury often exceeds 40°C. Some time in May or June, the southwestern monsoon brings rain and high humidity, cooking up a sweat for all but the hardiest of visitors. The wet season, which lasts until October, isn’t such a bad time to visit, as the rain tends to come in short, sharp downpours. Angkor is surrounded by lush foliage and the moats are full of water at this time of year. If you are planning to visit isolated areas, however, the wet season makes for tough travel.
Some visitors like to coordinate their trip with one of the annual festivals, such as Bon Om Tuk or Khmer New Year.
(Source: Lonely Planet)