I would do heavy pruning in January, but remember that that will stop flowering and fruiting for one to several years, depending on just how hard it was pruned. With light pruning, it may be easier to prune in early summer, when the first flush of growth is coming to an end, and the branches have sagged as much as they will. I'll admit, though, that I am extrapolating from plum tree pruning, since we don't grow cherries here!