South Africa is in the southern hemisphere, so summer runs from December to February. The complication is that the country has two opposing rainfall regimes: the Western Cape receives its rain in winter, and everywhere else in summer. There is no single best month, only a best month for the trip you are planning.
Safari: May to September
The dry winter is the reason to come for wildlife. In Kruger and the Lowveld, grass is short, deciduous trees have dropped their leaves and animals concentrate at the remaining water, so sightings are far more reliable. Days are 22 to 26 degrees and sunny; nights fall to around 5 degrees, and dawn drives in open vehicles are genuinely cold. Malaria risk is at its lowest. This is also the peak season for lodges, so book early.
Cape Town: November to March
The Cape summer is dry, hot and windy, with the south-easter blowing regularly enough to have a nickname, the Cape Doctor. Beaches, wine estates and mountain walking are all at their best. December and January are South African school holidays and the city is full and expensive. February and March are the sweet spot: the same weather, fewer people, and the grape harvest in the Winelands.
Whales: June to November
Southern rights are in the Cape bays from roughly June, peak in September and October, and have left by early December. This coincides with Cape winter, which is cool and wet but rarely cold, and with very low prices in Cape Town.
Flowers: August and September
After the winter rains, Namaqualand and the West Coast National Park erupt into fields of daisies and succulents for a few weeks. Timing shifts each year with the rainfall and the flowers open only in sunshine, from about ten to four. Check the flower hotline before driving four hours north.
Region by region
Garden Route: pleasant year round, wettest in spring and autumn, busiest in December. Drakensberg: green and stormy in summer, clear and cold in winter. Durban and KwaZulu-Natal coast: subtropical, humid and rainy in summer, warm and dry in winter, which makes it the best beach destination in July. Johannesburg: 1750 m above sea level, so summer afternoons bring spectacular thunderstorms and winter nights fall below zero.
School holidays and events
Mid-December to mid-January is the national holiday and prices double at the coast. Easter and the July school break are also busy. Fixed dates worth knowing: Freedom Day on 27 April, Youth Day on 16 June, Heritage and Braai Day on 24 September, and the Cape Town Cycle Tour in March, which closes roads across the peninsula.