I received my trees from the mail order catalog less than a week ago, and they're "leafing" very quickly. When should I generally move them into full sun to help them regenerate their sugars?
Bare rooted trees are normally planted at the end of winter or early spring before they break dormancy. I presume that you planted them into pots, and they woke up at the same time.
Since plant shock is mainly from transpirational moisture loss from leaves, and you don't have much in the way of leaves now, they can go into full sun now if it's appropriate for that tree. In the same way you plant into the ground bare root, you don't shift them to a sunnier location later on as the gradual change in light levels and temperature allows the tree to adapt.