Wait until fall if possible. Most of the roots are in the top 6 to 12 inches of soil, so go wide and shallow when digging. I once moved one that was only waist high and the roots extended to a 12' diameter so I root pruned in the spring, to a diameter of 6' and moved it in the fall. To root prune, dig a trench around the tree to a depth of 12" and fill the trench with compost. then fertilize with a fertilizer with a high middle # like triple super phosphate 0-48-0, to promote root growth. By fall the trench will be full of small feeder roots and you will have dictated the size of the rootball.