Yes, it does open up SEA-BOS and BOS-SEA. Mostly in winter. Late May includes Memorial Day weekend, but you never know. I fly it every two weeks and generally see it open about 3-4 weeks out, if there's any pattern at all.

Definitely use Expert Flyer (or a similar service) and call immediately if U does open up. The opportunity for U availability SEA-BOS-SEA is like a window in a mosquito infestation -- it opens and shuts fast.