We live in Vermont and in re: to seeing the best foliage in the VT/NH/ME area, the first week or two in October is usually when the leaves are at their peak. BUT...this all depends on the annual temps/rainfall etc. so one can never predict it to a tee. So, to be safe I'd say anytime from late Sept-mid October. Temps should still be relatively comfortable during the day...60's if the sun is out and at night it will be nice and crisp....bring a fleece jacket.
Have fun...there is nothing better than autumn in New England! Be sure to eat lots of fresh seafood, Mcintosh apples, apple cider donuts, pumpkin butter, maple syrup, freshly pressed apple cider, local cheeses etc...yum!