Cambodia has two seasons — dry and wet — and the difference matters.
November to April is the dry season and the peak tourist period. The cool season from November to February has the most pleasant temperatures — 24–30°C, low humidity, blue skies. This is the best time to visit if comfort is the priority. December and January are the coolest months and the busiest, with Angkor and Phnom Penh at their most crowded. February and March are still good: warmer but not punishing. By April, temperatures climb toward 35–38°C and the heat can be oppressive, particularly inland. Khmer New Year in mid-April brings celebrations but also means many businesses close and prices rise.
May to October is the wet season, brought by the southwest monsoon. Rain comes in heavy afternoon downpours rather than all-day drizzle — mornings are often fine. The countryside turns intensely green, the Tonle Sap swells to five times its dry-season size, and tourist numbers drop significantly. This means emptier temples, lower prices, and a more local atmosphere. Angkor in particular is beautiful in the wet season, with moats full and vegetation lush. Roads in rural areas can become impassable, and the far northeast (Ratanakiri, Mondulkiri) is difficult to reach.
October is a good shoulder-month compromise: the rains are easing, the countryside is still green, and crowds haven't yet built up. The Water Festival in Phnom Penh (October or November, depending on the lunar calendar) is one of Cambodia's greatest festivals and worth timing a visit around.
If you're going primarily for Angkor, any time works — the site is worth visiting year-round. The dry-season crowds at peak times (Christmas, Chinese New Year) are real; go early in the morning regardless of when you visit.