What a fun story, Mystery Author and really, perfect for the holidays.
I love that you start things off with Snape having become a non-entity after the war. It's not a take I've seen often but it really makes sense that the WW would be relieved to see the back of him.
I loved Lily as the ghost of Christmas Past, it allowed you to put in little bits of information which fleshes out the Lily/Severus friendship in a way we missed in the books.
Sirius and his big, honkin' piece of meat! Come on Snape, you know you want to bite it, lol!
The smut? Desperate, fumbling, ohgodI'vewaitedsolongforthis, wall!sex. One of my favorite flavors!
Poor Remus, Severus just couldn't resist letting him know that he watched him slapping the bishop the other night, could he? I love you Snape, you're such a jerk!
And the epilogue is perfect. I like the idea of the characters all having fun elsewhere and of course, Black's notion of a celestial threesome once Remus and Severus make their way to the other side. :)