The best time to visit Gaborone is May to September, during Botswana's dry winter season, when days are warm and sunny (20–25 °C), nights are cool (sometimes below 5 °C), and the risk of malaria in the city is negligible.
May–June marks the start of winter: vegetation has thinned after the rains, making wildlife easier to spot at Mokolodi and the Game Reserve, and the air is crisp and clear. July and August are peak months — expect dry, bright weather perfect for hiking Kgale Hill, and note that this is also the busiest period for safari-bound travellers using Gaborone as a base.
September is arguably the best single month: temperatures begin climbing back toward 30 °C, the bush is at its driest (excellent game viewing), and the city hums with activity before the first rains arrive.
October–November marks the 'shoulder' — hot (35 °C+), sometimes dusty, but dramatic thunderstorm light for photographers. December to April is the green season: lush landscapes, migrant birds in abundance, and lower accommodation prices, but afternoon downpours are common and dirt roads in the surrounding region can flood. Mosquito prophylaxis is advisable if you're day-tripping to wetter areas.