Angola has two seasons: the hot, rainy season and the cooler dry season known as the cacimbo. The best time to visit is during the dry season, from May to September, when skies are clear, temperatures are comfortable, and the roads are passable.
The rainy season runs roughly from October to April, though the timing varies by region -- it starts earlier in the north and later in the south. In Luanda and along the northern coast, expect heavy afternoon downpours from November through April. The interior highlands get the most rain, averaging 1,250 to 1,750 millimeters per year. Roads can become impassable, especially unpaved routes in the interior. Flooding is a real risk in cities.
The dry season (May to September) brings cooler temperatures and dry skies. Luanda's coastal humidity drops noticeably, and the highlands around Lubango can get genuinely cool at night -- bring a jacket if you are heading above 1,500 meters. This is the best time for wildlife viewing at Kissama, as animals congregate around water sources, and the best time for driving the interior roads.
The southern coast around Namibe and Benguela has its own microclimate, cooled year-round by the Benguela Current. Temperatures there are moderate even in the hot season, and rainfall is minimal -- under 100 millimeters per year in the far south, which is essentially desert.
June and July are the coolest months overall. September and October, the transition period, can be the best for photography as the light is dramatic and the landscape is still dry.
Angola does not have a tourist high season in any meaningful sense. You will not encounter crowds at any time of year. Hotel prices remain fairly constant, though business travel to Luanda peaks during the working months. Book Luanda hotels well in advance regardless of season -- availability is tight.