July to October is the classic Hwange season. The bush is dry, visibility is good, and wildlife concentrates around pumped waterholes. September and October bring the biggest elephant gatherings but also serious heat; start early, rest at midday, and carry more water than you think you need.
November to March is the green season. Rain disperses the animals and makes some tracks difficult, but the park becomes beautiful, migratory birds arrive, and many antelope give birth. This is a rewarding time for birders and repeat safari travellers, less ideal if your main aim is dense waterhole game.