The best time would be fall. You'll want to give them about 6 weeks to get rooted before your first frost. Hydrangeas actually prefer shade, but not full shade. They do best with a little morning sun or dappled sun throughout the day. They also prefer consistent moisture. The biggest problem with mophead hydrangeas is that the buds can be killed by late freezes, and they won't bloom. This is most common in the upper
zones like yours. You might want to consider replacing it with the newer Endless Summer kind that blooms on old and new wood.