If you're researching when to go to Rwanda then the chances are that it's for a gorilla trekking adventure. Although it's regarded as a year-round activity, the best time to visit Rwanda for a gorilla trek is during the short dry season from mid-December to early February or over the long dry season months of June to September. These periods offer by far the easiest hiking conditions and the lowest malaria risk.
You can visit Rwanda and trek gorillas outside these optimum times of year but bear in mind that the going will be more difficult in the rain as paths are steep and maybe muddy. It pays to be as fit as possible before you commence your trek.
The best time to go to Rwanda for chimpanzee trekking in Nyungwe, however, is during the two rainy seasons - mid-February to early June and mid-September to mid-December - as the apes are easier to locate. Food is harder to find in the dry seasons and the chimp families often range far into the forest interior.