While tracking or trekking mountain gorillas can be done any time of the year in either countries where (mountain gorillas are found i.e Rwanda and Uganda), because of the relatively wetter/ rainy seasons in the mountain gorilla inhabited national parks, many tourists often prefer the months from June to September and then December, January and February.

In Rwanda, the first rains start coming in late February through March, April and May and so many tourists try to avoid these months for fear of getting muddy and all wet while tracking the gorillas. However, during these months, because of the rains there’s plenty of gorilla food down slope and it’s much colder on the upper slopes. So the mountain gorillas keep on the lower slopes where they find it warmer and get food easily. So if you track the gorillas during these months, most likely your trek won’t be a long one – many trackers find the gorillas after 2 hours hike and by 2PM are back.
The same applies to Uganda although the rains there come in March, April and May. Of course the  Undeniably the rains (not always heavy though) make it rather harder to trek the steep terrains. Even then, the rain only comes for like 1-2 hours in like every 2-3 days and the sun comes out. We have noticed that even in the so-called drier months, the rain will often come in.

This seasonality of gorilla trackers can also be attributed to the weather seasons in the visitors’ home country and the nature of their jobs. When it summer for example in Europe, numbers of gorilla trek visitors increase and the gorilla trackers numbers dip when it’s winter.
However, there are other gorilla trekkers who often take advantage of the seasonal discounts on mountain gorilla tracking permits. Recently for example, the Uganda Wildlife Authority discounted the gorilla permits by 30% for non-resident foreigners.

Several accommodation and lodging facilities in mountain gorilla inhabited areas will also make discounts on their rates during these same months.

Whatsoever the case and explanation may be, experience shows that mountain gorilla trekking can be done any time of the year and you will most likely get the same the same experience.