The climate is mild yet varied throughout South Africa and there´s really not a bad time to visit. Weather and climate are influenced by a variety of factors from ocean currents from different regions, topography of the landscape, and coastal orientation.
Examples of the diversity are the possibility of snow between June-August in the mountain areas; rain in the afternoon during the summer (November-February) everywhere in the country except in the Cape Province, where rainfall is during the winter; not only is the rainfall during the winter it is also much lower and unpredictable leading to semi-desert conditions;
Those who go to South Africa specifically for animal viewing may want to go during winter -- grass is short, and the animals tend to gather around watering holes.