LOL I bought a package of 5 peonys from Costco last week also and asked what to do with them until I can get them in the ground...
I was told to get them in pots so thats what I did last week. I used a mix of seed germination potting soil and just straight potting soil and placed the peonys in a south facing window and I now have strong growth in all 5 pots. I only plan to keep them in pots until May when our last frost is.
If you can pot them in soil for the next month or so until your last frost date and keep them in a south facing window you will have a head start on growth and you can plant them in the ground in a short time.
I think if you wait and keep them in the store bought packaging until your last frost there is a chance the eyes may die off - that happened to me when I bought a pack of lily of the valley pips from Costco last year - I kept the pips in the orginal packaging and when I went to plant them they were all dead.
Hope that helps!

I'm no expert, but I've had great luck with potting peonies & planting in fall. I've found that if I keep them on the patio, I remember to water them because they're in plain sight where I can't just forget about them.
If you planted now and didn't let the plants "fry" & dry out in the ground this summer, they would probably do OK. You also have to be careful not to overwater as well.

There is absolutly no need to worry about spring frost on peonies. A hard freeze 20F or below might blight the flower buds but with new plants you are unlikely to have blooms this year. If you planted before the foliage appeared the ground temperature would keep the roots at the correct temperature until time to grow.

In PA and VA your soil should be at the temperature to start sprouting the earlier peonies. The later ones would start sprouting in about 2-3 weeks.

Other than you having time to plant or snow on the ground you can plant now. I have planted peonies in soild at all times of the year including winter as long as I could get a shovel in the ground.

Thanks for the advice! Since it's a bag of 4 maybe I will pot 2 & plant 2 and see what happens.

Another question -- since I am wanting these new peonies to focus on root growth, would it make sense to add some kind of phosphate-rich nutrient in the pot or ground before I plant it?

Personally, I find that digging & loosening up the soil 2x2 feet ( or even deeper if you feel up it!!)
if the soil's compacted (mine is crummy solid red clay with plenty of rocks) helps the roots get established. I add a handful of bone meal and some 6-month (not Miracle Grow) slow release fertilizer. I also mix in a bag of composted humus & manure and 2 bags of topsoil. The peonies seem to like that & many have bloomed a little the first spring after fall planting.
I forgot to mention if you pot some I would recommend a pot at least 12 inches deep. After a summer in a 12 inch deep pot, my peonies had roots coming out a little from the bottom of the pot by fall.

Thanks!
I have some Bulb Tone which has bone meal in it -- I can use that.