The best time to visit Zambia is during the winter months when the weather is dry and not too hot. Game viewing is at its most dramatic in October when it's very dry and hot in the valley. The rainy season usually starts in December, once the rains have set in by mid-January then the green season is at its optimum.
The winter months (British summer) is not only great from a weather perspective but a game viewing perspective. The summer months (British winter) can be hot and humid and there is always a reasonable chance of rain. Zambia has one rainy season which extends from December to the end of May.
A lot of the camps in Zambia are closed from the 1st December right through to mid-May on an annual basis, due to the roads being impassable because of the heavy rains. However, this is fast becoming a more popular time to come to those camps that are open - the park is quieter, greener and the river brimming to the full. It's a really popular time to come for regular safari-goers after something different, for keen photographers and bird watchers. There's a rumour that the game disappears and it rains all the time - this is not the case!