This is another one that I've done. For reference I'm in Lancaster, Pennsylvania, USDA zone 6b, last frost in April, First frost in September. We get 40-60" of precipitation per year, and the soil is mostly decent clay based soil, sometimes rocky. There were 3 of the trees and they were at about 12', so yours might be bigger. Because they dry and burn easily in winter, i took a trencher and just made an 18" deep trench about 35" in diameter, in the fall, after the temperatures cooled. Looking back, I probably could have made the rootball smaller, and been successful, because there weren't enough roots to hold it together 100%, once it was taken up. 

The trench helps it grow more dense roots around the edged of the rootball and can maker the transplant more practical as well as less stressful to the tree. The next spring, I hauled them out onto pallets when the ground was thawed out and the buds were just beginning to swell. I think that's the best time, for the least amount of stress to the plant. 

And as always with Sassafras, I planted them high (root flare 6" above the surrounding ground). I did not feel the necessity to stake the trees, since they were fairly stable and the area didn't get much wind. I used some general purpose fertilizer with the first watering, and then only felt the need to water one time after that, that first year, during a dry spell. They all grew well, and I saw lots of growth the following year. I wasn't back after that, but i haven't gotten any complaints so I'm assuming they did well.