September 15th is a little too early for fall foliage on the East Coast, for New York, Boston, and Philadelphia. Upstate New York (which is gorgeous during that time, by the way) in the Central New York/Southern Tier doesn’t peak until the middle of October, at the earliest. New York City is generally later, given its proximity to the ocean.
Boston would be earlier than New York, as it is further north, but still, coastal effects mean that you are not going to see brilliant foliage in the second week of September.