Zambia is a good place to travel to during any season.However the months from April along to the month of June and the again from August to November is the best. You get the nicest climate you can imagine
You can also visit here during winter.However, sometimes a lot of animalsgo for hibernation which may lead you to encounter only a few breeds. You will be missing that. Same with the months of monsoon as the rain will really spoil all the safaris which you may have wished to experience.