If it has buds set, I'd wait until after blooming. Moving/transplanting in bud will likely abort the flowering. But once flowering is done, you can replant in the ground. Just make sure not to plant too deeply -- root crown (with growing 'eyes') should be just below the soil surface.

Not a typical plant to include in a planter, to be sure :-)

Go ahead and replant it now if you want. It really doesn't belong in a planter. Even if it has buds now, the plant will be better off in the ground. In my experience peonies are not nearly as temperamental about transplanting as people think. I've transplanted in April and gotten a good flower show the same summer.

Do it when the foliage is firm and mature, such as after bloom. If you wait until September you can even just cut the foliage off for the year.

If you can dump it out and put it in the ground without cutting roots back or splitting apart the crown it will not be as affected as if you did one of those.

Since there were no buds, I planted it in the ground and it is doing very well. Thank you for your responses!