If you're going to Nepal for trekking, there are only two seasons - October-November and March-April. The former is best, when the air has been washed clean by summer monsoons and the passes haven't yet been snowed in. The weather is brisk but not too cold. In spring, the passes are open again, but the air is hazy, and the views, while good, aren't the same as in autumn. (If spring is the only time available, it's still worth going.) Because it's so far south, Kathmandu freezes only during the winter. Pokhara is warmer. The higher-altitude areas are quite cold, however. If you're going only to Royal Chitwan Park, be aware that it can be very hot and humid anytime during its season, October-May. The monsoons, which occur June-September, affect the entire country, and flooding is a possibility in the Chitwan region.