Ideally, in my area, when the tree is dormant. Here that means anytime the ground isn't frozen, or all winter minus the scattered days our ground is frozen. This gives the tree time to grow a strong root system over the winter.
BUT IN THE REAL WORLD, you know, where I really live... I usually find my trees mid summer in the big box stores. You know, the tree nobody wants, and is on clearance. I bring them home and plop em where I think is appropriate. So, I plant mine when I get them. If you are more organized, try to buy them in the fall.
The best time for me is when I have the money!
