September is the beginning of "shorts and hoodie" weather in New England. But the more north you go the sooner the shorts turn into long pants. But then we could get a "New England Summer in September where it jumps into the 80s. But nights do cool off quickly.
Basically, it depends. And it can change day to day. You will need to pack both shorts and jeans, light shirt and sweatshirt, light jacket to rain coat. I think pack like when going to Alaska. Think layers that you can take off and add back.
As the saying goes, "If you don't like the weather in New England, wait a few minutes". Especially in the fall.