Cambodia has two seasons and a narrow temperature range. What changes is rainfall and humidity, and the difference between the best and worst months is real but smaller than in mountainous countries.
Cool dry season, November to February
The peak season and the reason for it: days around 26 to 30 degrees, low humidity, clear skies and the countryside still green from the rains. Angkor at sunrise in January is cold enough for a jacket, which surprises people. This is also when everything is fullest and most expensive, particularly over Christmas and New Year.
Hot season, March to May
Temperatures climb steadily and April routinely reaches 38 to 40 degrees with high humidity. Walking around Angkor becomes a matter of starting at five and stopping by ten. Prices fall and the temples are quieter. Khmer New Year in mid April empties the cities as people return to their villages and involves being soaked with water in the street, which is welcome at that temperature.
Wet season, June to October
The south-west monsoon brings rain that falls in heavy bursts, usually in the afternoon, for an hour or two, rather than all day. Everything is green, the moats and rivers are full, dust disappears and light for photography is at its best. The Tonle Sap is at its fullest between September and November. The downsides are humidity, mosquitoes, some unsealed roads becoming difficult and island boat crossings being cancelled. September and October are the wettest.
Festivals
Khmer New Year, Chaul Chnam Thmey, over three days in mid April. Pchum Ben in September or October, when families bring food to pagodas for their ancestors over fifteen days, and much of the country stops. Bon Om Touk, the Water Festival in November, marks the reversal of the Tonle Sap with boat races on the Phnom Penh riverfront and draws enormous crowds.
Timing at the temples
Whatever the month, Angkor is a morning and late afternoon activity. Sunrise at Angkor Wat, breakfast, temples until eleven, a long break, then out again from three. The tour groups arrive at Angkor Wat mid-morning and at Bayon around ten, so reversing the standard circuit helps.
By region
The coast follows the same pattern with more rain, and is best from November to April. The north-east around Ratanakiri and Mondulkiri has its own microclimate, cooler and greener, and the roads there are worst in the rains.