In your zone I think I'd wait until fall to plant unless you can offer it plenty of temporary shade for the rest of the summer. Now that your bloom period is over the plant should start storing up nutrients for next bloom season. If you plant it now and the foliage dies back early because of it your roots won't be able to store as much. As long as the roots aren't too crowded in the current pot I'd leave it alone for now, just in a shady spot so it doesn't get overheated and water often, they like to be damp but not really wet. And fall planting will allow plenty of time for your new peony to establish its place in your garden.

I also like to add that you can plant the plant when it starts to go dormant. An indication of this is when the foliage starts to get a bit yellow. You do not have to wait until the foliage completely wilt before you plant. For your zone, this will probably happens in Sept. I suggest you cut the stems and foliage then down to about 2-3 inches of stems and discard those in the garbage and then plant it.

Are you in 7a or 7b? I am in 7b and have purchased many itohs in the past several years. They should be planted in partial or afternoon shade. I have planted as late as July and the plants have done fine. Cut them back to about one inch above the ground after Labor Day. They will begin to grow in late March or early April the next spring. Bloom in early to mid May. Enjoy the plant.

Always best to follow advice from someone in your specific zone! If you do plant it now make sure to keep it watered like any newly planted perennial. If it doesn't have the afternoon shade that graycrna4u suggested you might want to give it some artificial shade through this summer.