first they meet and she is sick, joking about the fact that she needs his blood and someone else's as well although it "fucks with your honor"
when he says "I knew it well", meaning he understood her, he knows what it means to keep a secret ("That secret that you know / But don't know how to tell)
second, they get involved in a relationship, and the secret becomes his as well (That secret that we know / That we don't know how to tell)
third, they started a family and the "Christmas morning creaks" may be those of a kid getting up, impatient to open up the presents
There are themes such as love ("you" becomes "we"), birth ("Christmas is associated with nativity") isolation ("secret") etc...
The fact that he repeats "I knew it well" throughout the song shows that he is remembering someone he felt empathy for.
The way I understand it:
when he says "I knew it well", meaning he understood her, he knows what it means to keep a secret ("That secret that you know / But don't know how to tell)
second, they get involved in a relationship, and the secret becomes his as well (That secret that we know / That we don't know how to tell)
third, they started a family and the "Christmas morning creaks" may be those of a kid getting up, impatient to open up the presents
There are themes such as love ("you" becomes "we"), birth ("Christmas is associated with nativity") isolation ("secret") etc...
The fact that he repeats "I knew it well" throughout the song shows that he is remembering someone he felt empathy for.
My two cents...