Due to multiple factors, expect to potentially pay more when booking in December. For low rates, March can be a great choice for travel to Pretoria. Flights to Pretoria from Newark Airport in March are about $974 on average during March, but can be found for as low as $669.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Newark Airport to Pretoria as you normally would.