Buy and plant in bloom. Otherwise, fall would be best, same as with all hardy shrubs.

Note that quite a few of the evergreen ones seem to have yellow foliage portions, this may be a virus problem. I have even seen established wild stands of the locally weedy D. laureola turning quite yellow on top lately. If something is sweeping through that will wipe this plant out here, great. If other daphnes are also going to become severely diminshed or problematis, not as great. It's not like daphnes are always the easiest shrubs to grow in the first place.