These are relatively drier months for the DRC in between the two rain seasons, so for visiting the jungles will be luch and green at this time and great for the birders as many migrant species will be here at this time.
March - April
This is a short wet period in the DRC so it can make being in the jungles a challenge but it does in some way make being in a jungle seem right with a bit of a rain. However for mountain gorilla viewing it can be very difficult climbing up the mountain sides and when viewing the gorillas they tend to become inactive and tuck themselves away to keep out the rain.
May to September
These are the drier and hot months of the year so considered the best times to visit. I suggest earlier in this period is better from a photographic perspective as the air is generally clearer which helps. Much of the energy year for households is from cooking fires so it can get quite smokey at this time. As the dry season takes hold the jungle does thin a bit as well so it can make viewing and climbing a little easier.
Oct - Dec
Another wet period in the DRC and as with other places in Africa it can be a amazing to be there when the first rains arrive and the transformation and awakening of many species is a spectacle to enjoy.