I've hunted there twice in May. First, we have to discuss "where"? Africa is a pretty big place.... Kind of like asking what to expect for weather in May in the US. Montana weather is a bit different, usually, than Arizona!
I will assume you mean RSA, or South Africa. Even there, there are regional differences. In the Limpopo region expect pretty cool nights, with daytime temps in the 70s. Very pleasant most of the time. It did snow on me once in Jo'berg in May, but was still mid 50's by afternoon.
Vegetation is generally still very green, making the bush fairly dense. Again, kind of like hunting the US in the fall before the leaves fall. A lot easier for the game to hide, but also the game is generally more relaxed. The rut for kudu and impala is probably on, depending on exact area.
Talk with your PH. I always go in early to mid summer because I have too much other hunting going on in the fall. Enjoy!
Bill