Hi' I have resurched this,and found one person that says aurtumn, and another that says early spring, but as this plant has a problem with late frost when the buds are on in spring,and young plants don't like it either, I am going with Bill Blackledge of BBCraido lancashire gardening program who advices late spring, the common name by the way is star magnolia,good luck

I would say spring as well. Magnolias dislike root disturbance at the best of times but spring will give them a whole growing season to make good any damage done at planting time

Don't forget to water it (rainwater if possible) for a whole year after planting, and stake it if necessary.