Usually right after Easter to avoid that early spring freeze that always seems to pop up.
From transplants, I am assuming.
I put the sweet potatoes (slips) in last year in June. They did unbelievably well. Based on the symmetry of their harvest/planting schedules, I would be tempted to plant the slips in the plot I just harvested by Spring potato crop in (plant in March, harvest June, plant in June, Harvest at Halloween). Not really crop-rotate-y, though. Hoping I can cure with a heavy backfill of compost after harvest.