The best time to move it is when it is dormant. Try to move as much of the rootball and as much of the tiny roots in the top 4" that you can. Give it TLC afterwards: maintain the soil well mulched and as evenly moist as you can. After moving it, the first summer should be one where it is very sensitive to heat stress so take that into consideration when watering or going on vacation.

Spring being the dormant period you mention. Before any new growth starts. Thanks.