When the best time of year to visit Mahale is, depends to a degree on what your objectives are. If your primary concern is to see the chimps, then while the obvious time to be there is during the dry season (June to October), there are good reasons for thinking about visiting Mahale in the low season, which means the time between November to March. The reason for this, is that the season is low because it doesn't suit humans quite so well as the high season. The same isn't true of the chimps who dont give a fig about the weather.
The bonus about going at this time of year is that it costs less, and there are far fewer people there. But speaking as one who has spent many months in Mahale during the rains, I can testify to it still being a superb experience for the chimps (and a lot else besides).
If on the other hand, you like the idea of more predictable weather and all the things that go with that, then it is worth paying the extra cost of going during the dry season - June - October.
My own favourite time to visit Mahale is the early part of the dry season (June – August) before everything dries out too much and the air becomes thick with that haze so typical of the late dry season. As a result the light for photography is beautiful and clear.
The shores of Lake Tanganyika are lined with deciduous miombo woodland and at this end of the year, the leaves are still on the trees and gradually turning, with a mini “fall" happening in July.
Weather wise the conditions are stable, with little chance of rain (other than in the mountains themselves) so the days are hot and dry, the perfect conditions to make the most of the beach and the water in Mahale.
Chimp viewing is of course the main event in Mahale and this is something that is never easy to predict. Because the food is fairly widely scattered through the forest, the chimps don’t follow any kind of a migration pattern. They simply go where there is the greatest likelihood of encountering food. Or sex. Better still, both.
So if chimp viewing is your primary concern, and you dont mind the chance of some rain, you would be well advised to look at the traditionally low season months of November to February, when the cost of staying here is so much lower than during the dry season.