Winter in the northern US is typically December through mid-March but it can be cold and/or snowy from about mid-October through April. The further north you go the worse it gets and the longer the winter season.
The extreme South and Southwest (Florida, Lousiana, the Gulf Coast, Southern New Mexico and Arizona, Southern California) basically never have a true winter but they can get rather cold (near freezing) on occasion.
Summer in the north normally extends from June through mid-September. The South and Southwest always have summer by northern standards.
If you are starting in upstate New York (Niagra Falls area) and want to avoid cold weather, mid-June is about the earliest you can rely on. Baton Rouge and anywhere in Florida will be very warm and humid that time of year. If you return to Pennsylvania by the end of August, you should be pretty much guaranteed warm weather for your entire trip.
Look here for typical conditions for almost anywhere:www.worldclimate.com
I hope your trip is all that you expect!