Answer by Rory Young:
Southern Africa has three main seasons.
The cool/dry season is from May to August. Because it is dry the game is easier to see as the vegetation is less dense and the game begins to congregate around water points to drink as puddles and other seasonal water sources begin to dry up. At the height of this season it can get very cold! I once drove down to Kruger and passed through snow on the Drakensberg.
The hot/dry season is September to November. The best time to see large game as the available water is by now very limited and animals are forced to concentrate around water-holes and dams. The bush is also very sparse and therefore animals are easier to spot.
The hot/wet season is from November to April. This is by far the most interesting point to see the ecosystem. With the first rains in October/November the impala and other species drop their young and then the bush steadily becomes more and more alive. A myriad of insects, reptiles and amphibians appear. Many migrant birds appear. It is however much harder work to find and spot bigger game and large numbers of animals because they are spread out through the bush as the water is everywhere. It is also difficult to spot game because the vegetation is very thick.
If you are interested in all species, incuding birds, insects and so on and are not that fussed about not seeing many "glamour" species then I would recommend the hot/wet season.
If someone is only keen to see larger and more exciting animals then I would not recommend the hot/wet season