The Northeast provides a plethora of activities, let alone places to visit and times to visit them in. Let's begin with when to go... The Northeast during Fall Foliage is the most beautiful not to mention the most popular time to visit!
With the changes in climate, it's pretty difficult to pin-point when winter actually begins, especially the more south you are from Canada. Based on past predictions, The best time to visit the northeast is October; however, based on statistical maps, there are different times that make the most difference.
Northeast United States stretches as far south as Maryland to as far north as Maine. Therefore, if you plan on taking in the foliage before Peak time, then you may want to start your trip while it's still Near Peak down south in Maryland so that you may see peak time near Maine.