klrguy: Why do you say they are obviously native?
For transplanting, yes, wait until late summer/early fall - but not too late, unless you are there to make sure it can be watered in nicely. Outside air temps should ideally be between 8-20degC. Water the tree a few days before digging it up, to minimize root stress. Dig out the rootball at a size of about 18" diameter + 12" deep and if you have to remove some of the outer soil with a trowel, then fine, but ideally keep as much roots as possible ad try not to damage them.
When you replant it, try to use as much of the original soil as possible in your new hole, as this will also help reduce transplant shock. Your new hole should be about 2x the diameter of the rootball and 1.5x the depth. After planting it in and making sure there are no big air pockets in the root zone of the new location, water it in slowly and heavily and make sure it gets regularly soaked until the ground freezes. Since it's your cottage and you might not be there as often to water it, mound up the soil about 8-12" away from the trunk in a circular fashion, to keep water localized to the root ball area when it rains or gets watered. You can also put a layer of some mulch down to help prevent the soil drying out, but make sure that it's no more than 3" thick and at least 3" away from the trunk to allow that area to breathe. Mixing in some organic matter (ie compost) with the soil in the new hole, will also help with nutrient and water retention.
nobbie48: For the Black Walnut - a 6" diameter tree is pretty tall and know that it has a taproot, so it can be a bit more involved than a tree with a fibrous root system. Without seeing it, I would guess that you would need to dig about 48" diameter rootball and about 18-24" deep, but also trying to dig up the entire taproot without severing it if possible. Then follow the tips above while making sure that the tap root is also well placed in the new hole.
Good luck!