Autumn is reckoned to be the best time for moving shrubs: the soil is a bit too hard right now, so I would wait until October unless you're feeling strong (or impatient?)! The main thing is that the Magnolia will be dormant (i.e. not trying to grow new roots).

is it in the ground or in a pot? If its in a pot them you can put it in the ground now if your soil is easy to dig. mine is a bit dry at the moment. if in the ground then I'd be tempted to do it back of October mid November provided it isn't frosty.

Answered the first time.

I'm going to disagree. Magnolias hate root disturbance, but early spring might be your best bet as any root damage would be made good just as it starts into growth for the new season.

Whenever you do it be aware that they have some very fine shallow roots so try not to damage them. I would try giving it a good mulch and see if that starts it growing.