Uganda sits on the equator which means that the weather is mostly like spring and is favourable all year round. Uganda basically has two seasons that run throughout the year; that is the wet season and the dry season.
The wet season is from March to May and October to November. The dry season comes in January to Feb and June to September.
Many tourists prefer the dry season and rightly so because you will be able to go gorilla and chimp trekking.
In the wet season you can still go trekking but it is harder to spot these animals and the path is very slippery making the trip longer. However this does not stop tourists from coming, there is still plenty to see here.
Unlike many of the stereotypes, the rain in East Africa isn't as drastic as many make it out to be. The storms do vary in magnitude but not enough to wreak havoc on your trip. Uganda's weather/climate allows for tourism to flourish all year round.
Easily the best time to come to Uganda is in the dry season since there is considerably less rain and you can enjoy your safaris and treks looking at the different wildlife in action.
December is a great time to come but it is a very crowded and the busiest time to visit because there are a lot of tourists in the parks.
Also the logging is usually booked solid for weeks. The most convenient time to come is September and February. These are dry months and you won't encounter as many tourists.
If you decide to travel in the rainy season be sure to get a good vehicle for the safaris because most of this terrain requires a four wheel drive to navigate. Alpha Rent a Car can provide you with the right vehicle and even offer you a driver/ guide to show you all of the hot spots of adventure.