June through August
December
In the summer if it's extremely busy. If you want scenery and enjoy the colors fall is always best however it gets cold up there really quickly. One minute you'll have sun and the next minute you will have snow. You should also take into account what kind of vehicle you're going to drive and plan accordingly. I typically take my motorcycle down and bring extra clothes knowing that the temperature fluctuates. I've been there several times in the spring, the traffic isn't so bad up there it's easy riding. No real worries about oncoming traffic in early spring.
Depends. Leaves changing color is awesome. Spring and fall are generally better in my opinion because there's fewer tourists.
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