In USDA 5 you would probably be best doing it at end of winter/earliest spring, after coldest weather but before bud break.

From the photo it appears that you have a small Acer palmatum dissectum. It is true that you will do minimum damage to the plant by moving it after the leaves have dropped, in the Fall, when it is dormant. At this point, the plant's energy reserves/food supply for its Spring flush is stored in the root structure.

By the size of the plant I would suggest digging a root ball about 1 ft in radius [two feet across] and at least 1 ft deep. Prepare the new hole first, a bit larger, with some fine filling soil on the side. Once you dig the tree, move the root ball quickly, minutes count, to the new location. Make sure it is a snug fit in the new hole by adding filler soil around the perimeter, then light pess into place with your foot. Give it a single watering just to settle loose soil and remove any large air gaps.

I would follow Ron`s advice and do it in the early spring. Kuhn`s advice in cutting around the root ball now would also be good to do and let it sit till spring.
Sna

I like to transplant them as soon as the sap is going down and with the leaves. For me this is about 2 weeks before you expect fall color. I have dug them at that time and within 2 weeks I get dozens and dozens of new white roots that are already 5 inches long.