Roughly speaking, the summer months are December to March, autumn is April to May, winter is June to August, and spring is September to November. Because southern Africa is such a large area, and each region's offerings change with the seasons, when you go may determine where you go.
The Coast -- South Africa's southwestern coast (the province known as Western Cape) tends to attract the majority of international visitors during the summer months. Fortunately, the country is big enough to absorb these increased numbers without causing the discomfort most people associate with busy seasons, though parts of Cape Town become unbearably full for locals' tastes. Be aware, however, that accommodation prices do increase in summer, some by as much as 70%; and if you dislike crowds, you should try to avoid South Africa's coast during the busiest school holidays, which -- like elsewhere -- take place from around mid-December to mid-January. In fact, the hot months of February and March are considered by most Northern Hemisphere dwellers to be the best times to visit; not least, to escape what by then has felt like a very long winter up north. April, too, is a great month to visit, when the light takes on a softer hue, sunsets are often spectacular, and balmy temperatures are preferable for those who dislike baking heat. Note, though, that you'll need to book early if your visit coincides with Easter weekend, when you will compete with locals on their 10-day school vacation.
Depending on your interests, winter (June-Aug) brings substantial benefits, too: July to November are the months when the Southern Right whales migrate to the Cape's southern coast, providing the best land-based whale-watching in the world (and plenty of opportunities to see them by boat). With the Cape a winter rainfall area, local tourism authorities have aptly dubbed the May-to-August period its "green season," and indeed the Cape's valleys and mountains are an ideal verdant backdrop to dramatic displays of fynbos (shrublike plant) in flower. While it can at times rain continuously, the pattern is usually broken every few days with balmy, sun-drenched days. It's a wonderful time for Capetonians, who get to air their winter coats for only these few months and reclaim the city, now virtually empty of visitors, as their own.
If you're at all hankering for the sun, plan to head to the Garden Route and Karoo, preferably along Route 62, where year-round sunshine ensures that any time of year is a great time to tackle a driving tour of this region. Winter is also the ideal time to visit the east coast of Kwazulu-Natal, which can be oppressively humid in summer.
October to November is when the Cape floral kingdom again wows her human inhabitants with a new batch of flowering species, while the beaches, still relatively empty, sparkle in the temperate sun, and guesthouses and hotels, hungry after the winter wait, offer some of the best deals of the season.
Inland -- May to August are considered the best months for sighting big game in and around Kruger National Park. The foliage is less dense, malaria risk is lower, yet many of the private game reserve lodges drop their prices substantially.
June to October, however, is peak season in the Delta, Botswana; game-viewing is best at this time, thanks to the rains. Given the low-volume approach to tourism, lodges are booked months in advance, despite charging top dollar. The Delta is often combined with a visit to Victoria Falls, but the Falls are widely considered most impressive in full flood, between March and May, when some 500 million cubic liters of water cascade into the Batoka gorge every minute. The spray can obscure views, however, and prohibit riding the Batoka's Grade 5 rapids -- renowned in rafting circles as one of the most exhilarating rides in the world -- as they are out of bounds when the falls are in flood.
Depending on where you are, average maximum temperatures can vary from 80°F (27°C; Cape Town) to 90°F (32°C; Kruger National Park) in the summer, and from an average 69°F (21°C; Cape Town) to 77°F (25°C; Durban) in winter. While summer is the most popular time, high humidity in KwaZulu-Natal can make for muggy days, and gale-force winds often occur in Cape Town and Port Elizabeth. Winter visitors would be well advised to pack warm clothes, despite higher average temperatures than in the United States or Europe. South African buildings are not geared for the cold; insulation is low on the priority list, and central heating is nonexistent. Temperatures in the interior fluctuate wildly in winter; you're best off layering.
South Africa is generally considered an arid region; two-thirds of the country receives less than 500mm (20 in.) of rain a year. In the interior, rain usually falls in the summer, and spectacular thunderstorms and the smell of damp earth bring great relief from the searing heat. The Garden Route enjoys rain year-round, usually at night. In Cape Town and surrounds, the rain falls mostly in the winter, when the gray skies are a perfect foil for the burnt-orange strelitzias, pink proteas, and fields of white arum lilies -- and an equally good accompaniment to crackling fires and fine South African red wines.
If you are traveling during the South African school holidays, make sure you book your accommodations well in advance. (Check exact school holiday dates with South African Tourism, as provinces differ, but they usually run 4 weeks in Dec and Jan, 2 weeks in Apr, 3 weeks in June and July, and 1 week in Sept.) Flights can also be impossible to find, particularly over the Christmas holidays. Easter holidays (usually late Mar to mid-Apr) can also be busy, while the Kruger is almost always packed during the winter vacation (mid-June to mid-July). There's another short school break in spring, from late September to October 7.
Zimbabwe and Zambia's climates are similar to that in South Africa's northern provinces, with a rainy season in summer, mostly between December and mid-March. Summers are warm to hot (late Oct-Dec can be uncomfortable), and winters are mild. Malaria is still a danger in many areas. There are tsetse flies in parts of the Zambezi Valley and in the southeast. And certain rivers, lakes, and dams are infected with bilharzia. Victoria Falls are often at their fullest around mid-April, at the end of the rainy season (Nov-Apr), though this is also when the mist created by the falling water may obscure the view and malaria-carrying mosquitoes are at their most prolific. Temperatures range from 90°F (32°C) in October and November to 60°F (16°C) in June and July. Many think the best time to see the falls is from August to December, when the view is clearer (though the flow of the water is at its lowest). June through December is high season for many of the upmarket lodges, which raise their prices during these months. You are unlikely to be affected by public holidays.
Botswana has a pleasant, temperate climate with low humidity, with a maximum mean temperature of 91°F (33°C) in January and a minimum mean temperature of 38°F (4°C) in June. There are effectively two seasons: summer (Sept-Apr), with frequent rains and thunderstorms, and winter (May-Aug), with cold and dry days and nights. Rainfalls make the summer months a great time to visit the delta if you're interested in birds and plants, but it can get very hot. From April to September, the days are mild to warm, but temperatures drop sharply at night and early in the morning, particularly around June and July. Most consider these 2 months the best time to visit the delta, when the rain that falls on the Angolan bushveld plains seeps down to create what is referred to as the "flood." At this time, water lilies bloom, countless aquatic creatures frolic in the water, and a huge diversity of game from the surrounding dry areas moves into the delta. You are unlikely to be affected by public holidays.