In the spring !
Are you referring to shrubs or perennials ...... either way, the answer would be the same.
Doing it 'then' allows the roots to settle in a bit before the warm/summer (?!) weather puts too much of a strain on their roots/water uptake.
And, when you do it take as much of the rootball as you can because this will make the plants ability to establish so much better, take too little and you'll either lose it or lose a seasons growth.

but not if the ground is frozen or if frost is forecast. otherwise i fully agree with louise.