Rwanda can be visited at any time of the year. The long dry season, June to September is the best time for taking safaris in Rwanda. The dry season is the best time to go gorilla tracking in the Volcanoes National Park and for hiking in Nyungwe Forest, since the ground should be dry underfoot and the odds of being drenched are minimal. This should not be a major consideration for any reasonably fit and agile travelers unless they are planning to hike to Virunga peaks such as Visoke or Karisimbi, in which case the rainy season should definitely be avoided. The dry season is also the best time for traveling on dirt roads and it is when the risk of malaria should be lowest.
There are two annual rainy seasons, the big rains which last from mid-February to the beginning of June and the small rains from mid-September to mid-December. Rainfall, especially over the mountains, can be heavy during these two periods particularly from March to May, although it is still perfectly feasible to travel at these times of the year and for those visiting at short notice, it is far easier to obtain a gorilla permit at the last minute.
As for the two dry seasons, the major one lasts from June to September and the shorter from December to February. However, the climate is not uniform throughout the country. It is generally dryer in the East than in the West and North. On occasion, the volcanoes of the North may be capped by snow and evenings in Kigali can call for a sweater as do days anywhere in the highlands should you happen to hit a cold snap. Nevertheless, every season is good for swimming and tanning on the banks of Lake Kivu. An advantage of travelling during the rainy season is that the scenery is greener and the sky less hazy at least when it isn’t overcast, a factor that will be of particular significance to photographers. The wet season is also the best time to track chimps in Nyungwe, in the dry season they may wander further off in search of scarce food, while the months of November to March will hold the greatest appeal for birders, as resident birds are supplemented by flocks of palaearctic migrants.
Rwanda is small and all parts of it are so easily accessible from Kigali that you needn’t engage in any complicated planning. Your first port of call should be Kigali, to gather information and to get your gorilla-viewing permits from the ORTPN. It’s sensible not to rush off to the gorillas immediately, take a few days to get the feel of the country and to acclimatize, because the trek can be quite strenuous and the altitude can catch you unwares.