If you live in an area that gets winter when the plant is dormant. This will be the best time, before the tree starts to bud.
Pruning actually is not necessary.
If its already leafed-out, then wait until the berries have ripened and prune it lightly, in the summer, using the Rules of Thirds. Don't cut any branch back more than 1/3 of its total length.
I prune mine before it gets too hot for the summer. I actually did it yesterday, and will not prune again until November.