Due to multiple factors, expect to potentially pay more when booking in December. For low rates, October can be a great choice for travel to Johannesburg. Flights to Johannesburg from Chicago Midway Airport in October are about $949 on average during October, but can be found for as low as $704.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Chicago Midway Airport to Johannesburg as you normally would.