When to go on safari is sometimes determined by what animals you would like to see. The best time to go on safari to see a wide diversity of animals is covered on page 1 and page 2 of this article. But if you would like to plan your safari around seeing gorillas, chimpanzees, birds or whales, it's important to time your trip perfectly.

Gorillas

Gorillas are actually a year-round attraction since their habitat has been reduced so significantly, they couldn't roam far even if they wanted to.

However, tracking gorillas is hard at the best of times and during the rainy season, the steep paths and mud can make it almost impossible to manage. Very heavy rain also makes it more difficult to take good photos, and since you only have an hour with the gorillas, it would be a shame not to get a good snapshot or two. The main rainy seasons in Rwanda, Uganda and the DRC are from March to April and October to November.
More about gorilla safaris....

Chimpanzees

Chimpanzee safaris can be found in Western Tanzania and Uganda. Like gorilla safaris, they can take place year-round but the rainy season makes walking in the forests a little tougher and the photo opportunities aren't as good as in the dry season (July - October and December). However, the rain also means the chimpanzees don't have to roam too far to find water and they are easier to locate (February-June, November-mid December).

Whales

South Africa offers some of the world's best whale watching especially if you don't fancy going out on a boat, but would like to see them from the shore.

The best time to watch whales is from June to November when the Cape coast comes alive with hundreds of southern-right whales. You can also see humpbacks, Bryde's whales and orcas.

Birds

The best time to see birds in Southern Africa is between November and March. South Africa, Namibia, Botswana, Angola, Zimbabwe, Zambia and Malawi are all excellent destinations for birders and many birding safaris are available.
In East Africa the best time to go birding is January - March. Kenya, Tanzania, Uganda and Ethiopia are all popular birding destinations.

West Africa also offers a huge and exciting variety of birds, the best time to visit Cameroon, the Gambia and other destinations is during the European winter from November to March.

See my Safari Planner for information on the best destinations to see the Big 5 (elephants, rhino, leopard, buffalo and lion), crocodiles, hippos and more.