Like most of East Africa, Uganda is warm and dry from roughly June to September and then again from mid-December to the end of February, and these are probably the best times to visit. The game viewing in the national parks is marvellous from June to September and gorilla trekking in these months is lovely thanks to slightly cooler temperatures. December, January and February are just as good for both (perhaps a little hotter) and sometimes quieter which can be a bonus. In the in-between months (November; March to May), you can expect more humidity and rain showers. You can absolutely still visit but there may be less choice in accommodation as some of the more remote places become inaccessible. Zanzibar and the Kenya coast follow Uganda’s dry seasons so if you’re visiting at this time, a beach break at the end is a good addition!