It’s generally more expensive to depart on a Sunday, if you have to leave on this day you can still search on Cheapflights to find great options that meet your needs.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Johannesburg to Redding as you normally would.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Johannesburg is roughly less than Redding. You can assume this to be the case over the next 19 days.
With an average temperature of 38 °C, peak weather can be found in July. As a traveller, expect to see changes in flight prices during that period. The temperatures in Redding are hotter than in Johannesburg.