Now. Take a large root ball to disturb as little as possible and then keep well watered when replanted. Don't do it when there is a frost or likely to be one a few days after planting.
I agree with Hawkwalk, not always successful moving conifers, so beware!! Try to avoid damaging roots as much as possible. You may notice a white mildew on the roots, do not clean it off!!! The tree has a symbiotic relationship with this mildew looking fungus (symbiotic = they need each other to survive)