The best wildlife-viewing months in Rwanda are during the Dry season, from June to September. Primate walks in the forest are a big part of any safari in Rwanda. The habitat of rainforests is, by default, very wet and one can’t avoid rain completely. Walking in the forest is easier in the Dry season and that is also the recommended time for most primate walks, including gorilla trekking.
June to September –Dry Season
Wildlife Photos

October to May –Wet Season
Wildlife Photos
Best Time to Go to Rwanda by Major Park
Overall the parks are best visited in the Dry season, from June to September. Chimpanzee trekking in Nyungwe Forest is marginally better in the Wet season, but that advantage could be countered by the very slippery trails. General hiking and other primate walks are best in the Dry season.