Namibia is in the southern hemisphere of the earth, which means the seasons are reversed. Summer in Europe means winter in Namibia. Winter in Europe means summer in Namibia, etc.
As a rule, summer in Namibia is from October to April.
During this time it can be over 30°C a day, but it is also the rainy season. Usually it rains from December to February. The rain showers are comparable to summer rains in Europe. Still warm and sometimes they are over faster than you thought. So you should think of all possible weather conditions during this time! Don't forget your sunscreen, but don't forget your rain jacket either ;)
Autumn is quite short from April to May.
During this time it gets a bit cooler, the landscape is lush green and the sun sets much earlier.
Winter is usually from May to September. During the day you have +-20°C. It is the dry season, so you don't have to expect rain. At night, however, it can get cold. In some regions of the country, temperatures can drop below zero! So pack a warm jumper and some warm socks. Plan to spend a few nights in a lodge instead of camping in the cold. It's also much windier in winter than usual, so contact lens wearers might also want to switch to their glasses (applies to myself and definitely can't do without them at this time!).
Spring is very short. In September, a lot of things are in bloom and nature slowly becomes more colourful again. The temperatures are rising again and it is generally very pleasant to travel at this time. It is neither too cold nor too hot.
The weather also varies a lot from region to region. While it tends to be warmer and drier inland, it is often foggy and a few degrees colder on the coast. It is best to check what the weather will be like in your travel period before departure and pack your suitcase accordingly.