Snape survives DH, but ends up in a coma. The ministry is paranoid and orders strict observation for all former Death Eaters - even Snape. The task falls to Percy, who finds himself caring without knowing why. Then Snape wakes up...
Severus doesn't like the idea of Christmas dinner with Percy's family.
When Severus is sick, Percy makes a discovery that won't leave him alone. Later, it proves to be just the help Percy needed to make Christmas unforgettable for both of them.
The first three times, Weasley comes to him for one thing only. Or at least that's what he tells Severus.
Starting with a detention in his first year at Hogwarts, Percy over the years gets to know a side of Snape most other people have no idea exists. Why does he trust him even after the events of HBP?