Assuming that it is of a size that can be moved, you could do it anytime now that it is dormant, as long as the ground isn't frozen. Pick a spell of weather that looks milder and prepare the hole for where the tree is to go. Dig a hole plenty big enough and improve the soil with compost and add bonemeal or similar. Then, dig out the tree with as much rootball as you can manage. Get a friend to help! Move it on hessian or old compost sacks. Place it in the prepared hole (you'll be glad you did this first!) and refill up to level that it was at before. Firm it in. You may want to put a stake in before backfilling. Water it in and keep it well watered for at least a year.