Best months are September and October. The earlier in the year the farther north you will have to go. On average the colors start about mid-September in the north country and gradually work their way down for the next 6-8 weeks. There are so many variables it's impossible to know timing more than a few weeks out. I will say you should make reservations as soon as you know your schedule. Leaf peepers take up a lot of space. Kids are back in school by Labor Day but weekends get crazy.
It's also fair season up here that time of year. There are too many to list so I'd get on the tourism websites of the states you want to visit. That also adds to the craziness as fairs like the one at Eastern States Expo in Springfield, MA, can take more than one day to fully appreciate. The list of "must sees" can also be endless so if you give more specifics about what you like to do, whether or not there will be kids with you, etc., that would help. Not only is the foliage beautiful in New England but the Great Smoky Mountains and other more southern areas can be spectacular.
I may not have gone where I intended to go, but I think I have ended up where I needed to be
Douglas Adams
[purple]RV-less for now but our spirits are still on the open road. [/purple]