Departures are available from Boston on Sunday through Wednesdays, with returns from Porto valid on Tuesday through Fridays. There will is a 7-night minimum stay required for this fare, but we think you'll find plenty to do over that week to keep busy.
Searching on United, I could find seats departing BOS on United on Tuesday, September 18 and returning from OPO on Swiss via Zurich on Wednesday, September 26, 2018.
There are some other fantastic fares from Boston like the one we mentioned earlier to Nice for $420 round-trip. You should make sure to bookmark the Boston (BOS) flight deals page so that you won't miss out on another discount deal to Europe.
If you need a place to stay in Porto, Hotelwatchdog has a variety of options available for the mentioned flight dates above.