The fall after it is dormant!
That being said, I've moved many larger plants in the middle of the summer. The key is getting a root ball big enough that the plant doesn't know it's being moved.
Pia,if you only planted it last spring you may well get away with moving it now. Take as big a root ball as possible, and prepare the planting hole in advance, so that the tree is out of the ground for as short a time as possible (it might not even notice being moved, as OED says ) and water well in