The best time to visit Zambia is during the winter months when the weather is dry and not too hot. The rainy season usually starts in December.
The winter months (our British summer) is not only great from a weather perspective but a game viewing perspective. The summer months (British winter) can be very 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.
Most of thecamps in Zambia are closed from the 1st December right through until the 1st May on an annual basis, due to the roads being impassable because of the heavy rains.