South Africa is an all-year round destination due to its varying regional climates. Between November and February, The Cape has beautifully hot and dry summers, while the best time for whale watching is between July and November. The northern regions can be rainy from November to February, but on the other hand it can be marvelous for birding. The cooler winter months (May to September) are best for spotting amazing wildlife. So, it really depends on what you want!
If you decide to visit South Africa in between December and February, keep in mind that it’s also school holidays here, so it can be very busy.