Most people recommend late Fall when they are dormant. But my favorite time is when the new shoots first show above the ground in Spring. I find that the bulbs have very few roots on them then, and are very easy to transplant without setting them back. If you wait too long, none of this is true. If you garden in heavy soil, it might be better to transplant in Fall, before the soil gets too wet and cold. Another strategy is to make raised beds in Fall and cover them with plastic. That keeps the soil in perfect condition for planting all Winter long and into Spring. But I think the best plan is to improve heavy clay loam soil with lots of coarse sand. You can then work in the soil at nearly any time of year, and the plants grow amazingly well in it.