The best time to visit Zambia is traditionally during the winter months between June and October/November. This is typically the dry season when days are bright and clear and temperatures pleasant during the day, although they can be quite cold at night. As the season wears on, the grass dies back and the foliage thins out, making it easier to see the animals. Furthermore, water becomes more scarce so wildlife concentrates around the remaining water sources.
That said, if you are a keen birder or particularly interested in the vegetation, you may want to consider visiting during the green season. Some areas (such as the Lower Zambezi) close altogether during the rains due to poor access and flooding, but the South Luangwa is a lovely place to visit at this time. The summer is also a great time to see the young being born, migrant birds and birds in breeding plummage, not to mention glorious floral displays as the trees, grasses and wild flowers go bezerk. Obviously there is the chance that you may get rained on during this time.
Posted by: Alex