As an example, I found tickets at this price departing BOS on Wednesday, May 1, returning from MAD on Thursday, May 9, 2019.
Seats are starting to sell out fast, but you should be able to see other dates available for travel in early to mid-May. A minimum stay of 7-nights is required.
This is a Basic Economy fare, which only includes carry-on bags and does not include advanced seat selection. A checked bag costs $60 each way. The upgrade to a Main Cabin Economy fare, which includes one checked bag and standard seat selection is $180 more roundtrip.
Need a hotel in Madrid? Here's what's available for the above dates on Hotelwatchdog.