The best time of year to visit South Africa, with regards to weather, depends on where you are going. Durban has warm weather nearly all year round, Cape Town is cold and rainy in winter and Jo'burg is hot and stormy in summer.
Nenita, I suggest you get a met by a taxi or shuttle service and get taken direct to Pretoria (Tshwane). You dont want to get lost in a hirecar at night in JNB or Pretoria.
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Generally speaking, the summer months are September - April and the winter months are May - August.
Perhaps you could let us know what your travel plans are for South Africa (sightseeing/ activities) and we can help you out a bit more.