It's never too early to start planning your holidays for next summer, especially when its a sub $500 fare for peak travel in July and August. What's even better about this fare price is that it includes a piece of checked luggage. Base prices on TAP Portugal are a bit lower but when you begin to add on extras, the fare ends up almost $100 more than Delta's deal.
As an example, searching on the Delta website, I found tickets at this price departing BOS on Thursday, August 1 and returning from LIS on Sunday, August 11, 2019. Departures are available Sunday through Wednesday, with returns available any day of the week. There is a 7-day minimum stay requirement, but other than that this fare is widely available. I'm not sure how long it will stick around or if it's an introductory price, allowing Delta to feel out the market with this new service. If you're interested in this deal then I'd recommend booking now, as we rarely see full-service nonstop summer flights drop this low.
If summer 2019 its too far off for you to start planning, no worries, we have a full list of other deals departing Boston (BOS) here for you to look at.