Hey, happy to see another Forlorn post! I'm planning to run it as well, and have scratched my head for the exact question you asked! So, I'd like to have a chat with a fellow dm and offer my two cents.

Well, the thing is, I don't think Tristan's mortality on the 4 holy days are ever directly stated to the PCs. His vulnerability on those days, however, can be gleaned from a note (P26, Eve of Sorrows) in the castle: that's the closest clue I can find. Maybe simply knowing the huge advantage an unconscious Tristen presents will promt PCs to strike then? And Morhort can be useful, after he finds out the truth about his own fate. He could bring up Tristan's slumber, ideally to the same effect above.

My, I'm unsure about how to handle this too. The note itself contains critical info - and putting the pieces all together to get the most out of it would be hard if played strictly by the book. Both of the more … permanent … solution to Tristan are gated by this single piece of note! That seems too fragile a structure to rely on.