Late Fall and early Spring. However, my opinion, as the Russian army is now largely dismounted, the mud season is no longer a huge problem. It is a problem for large vehicles, but a bit less of a problem for soldiers on foot, as it were. The soldiers won't like it, but they can keep moving, slowly.
Instead of pulling a small 2-wheeled cart behind a soldier loaded with a small amount of supplies, they can pull a flat-bottomed sled instead, as would be used in winter. A walking army can keep going in almost any weather. And drones of course keep flying.
The Russian soldiers will have less tank and artillery support, due to the mud, but that support can be largely supplanted by drones. So the ground war will continue in modified form.