It would be best to do this as soon as the conditions are better..ie.not frozen or threatening bad weather. So pick a mild spell and remember to prepare the receiving hole first (lots of compost and some bonemeal), then you won't have to leave the tree out of the soil for long. Don't forget to water it in and water it all through this year. Oh, and put a good stake in the hole and use a proper tree tie to keep the tree steady until it is well established.