The question of when to go to Botswana is best answered by thinking about what you want to experience. Game viewing is at its peak during the dry winter months of May to October when animals are concentrated in ever increasing numbers at water sources as the dry season wears on. Many experts regard the cooler months of June to August as the best time of year for a Botswana safari - the game viewing is consistently excellent, there's virtually no rain and the risk of malaria is at its lowest.
Reaching peak concentrations during the hot and dry September/October months, wildlife tends to disperse during the rainy January to April summer but several destinations such as Chobe's Savute region and the Kalahari offer excellent summer game viewing, not least because they lie on the path of migrating animals - particularly zebra. Indeed, Botswana's three Kalahari parks are arguably at their best in early summer - December is a particularly good month as many antelope give birth then which means more predators.
A year-round birding destination, bird watchers will nevertheless find the the migrant-filled summer months of the "Green Season" the best time to visit Botswana for both numbers and diversity of species.