Bare root trees are best planted "as soon as you can" in early spring. There's a generous window here as far as meeting conditions like mud and having volunteers available goes. Ideally, root systems are in the soil before much happens as regards buds swelling. We're not emulating the maple tree here -- early sap flow is one thing in places where the land freezes but then comes a point when you stop tapping and boiling down sap as maple buds start to swell. Fruit trees are a beat or two behind this progression but silver tip does inevitably come. This latter period can last 4, 6 weeks easily ... which for Nathan in north-central Pennsylvania is probably mid-March through the entire month of April. And if winter gets prolonged, those first weeks of May may be in play as well.
Planting once green tissue shows and leaves start to grow works too. Trees will live, albeit need more fuss as regards initial watering if it's a dry spring. That said, the later you go, the more wood growth on that first season will be set back. Shoot extension may have been 2 plus feet heeding the advice ... now it's more like several inches, and thus good growth doesn't begin till year two. You basically lose a season if you're a laggard ... which happens despite best intentions sometimes!
The trickiest part of this is having trees on site when you deem the time is right. Mail order nurseries need to avoid the stock getting frozen in transit, and so predict the proper delivery time for your location. But in a very warm spring this may hold you up.
Lost Nation OrchardZone 4b in New Hampshire