Fall or spring are best.
Fall, before frost, and....you get some really GREAT deals on left over plants that look orphaned. But they always have tags attached so you can see what they are and require. You would plant, water well and mulch.
Spring planting after the frost.
Your area will sell what's good for your area when the time is right.
What are you thinking of growing? One of my favorites: Sonya Roses.
We did a great thread last summer. Type in "Who Knows About Roses" in the search. Maybe you'll find some info there that will help you.