Flights at this price are available for departures Sunday through Tuesday starting in mid-January through early March 2019. Other weekdays are only a few dollars more if you want to travel in the fall you can find travel from November onward for about $20 more. A Saturday night stay is required.
As an example, I found seats departing JFK on Monday, February 11 and returning from LIS on Friday, February 22, 2019. Perfect for a romantic Valentine's getaway.
If another destination in Europe is on your mind, make sure to have a look at our newly updated Europe Flight deals page where you can find a full list of fares across the pond.
If you need a place to stay in the Portuguese capital, here's what's available for the sample dates listed from Hotelwatchdog.