I would think anytime between autumn and spring provided the soil is not frozen or waterlogged. Having said that I moved a rowan tree in July a couple of years ago and it's still here to tell the tale!Prepare the planting hole first, then lift the shrub with a ball of soil and plant so that it hardly realises it's being moved. With the rowan I had to chop through some of its roots so I pruned the leafy crown too and gave it water by the bucketload.