For most people the dry (winter) season is most attractive (i.e. April/May to October/November). The chance of rain is slim, daytime temperatures are generally pleasant (in the 20s Celsius) and the low vegetation and limited availability of water mean that game viewing is at its best.
Rainfall is extremely rare in the dry season and even in the so-called wet season, the rains are usually short-lived storms, as is typical of the tropics, and at no time does the climate seriously inhibit the traveller