Totally unexpected.

This is a very sad, yet lovely story. I like the original idea and the use of music throughout added extra interest.
I thought that the use of ‘her’ during the first section of the writing instead of giving away that it was Luna was a great idea. It gave an air of mystery to both the girl and her music, which I assume was intended. Well done!
Your characterisation of Harry was very consistent with the books. I can just imagine that he’d do rounds at St Mungo’s on Christmas Eve in the aftermath of the war. Luna, too, was well written in her very expressional and deep-thinking manner. Finally, the relationship between the two characters was sufficiently awkward at the beginning, developing into quite a tense section, followed by anger. This progress through the emotions really allows the reader to pick up on the background to your story, despite it being a one-shot.
I especially liked the use of the sentences:

“Harry rested his elbows on his knees and ran his hands through his hair.”
Giving a great sense of Harry’s despair at the words he is hearing.

“He’s green eyes flashed black, and he slammed his fist on the piano keys.”
Displaying Harry’s anger and frustration at Luna in the present, but also his frustration at the fact that many Deatheaters remain uncaught.

I did notice a couple of slight things that I wanted to point out if it doesn’t seem discourteous.
1. “He’s green eyes flashed black, and he slammed his fist on the piano keys.” I think that the ‘he’s’ was meant to be ‘his’?
2. “It’s not as if I’ve had to deal with?” I’m afraid I got a little confused when reading this part, and thought it may be missing a word.
3. “Because even though I believe you could right about him…” I thought that this sentence may read better as “Because even though I believe you could be right about him…”

Other than these tiny little things, it was perfect! I thought that the ending was very fitting with the story. It left a sense of the unfinished yet with no need for another chapter to be added. “While the note now held different meaning, it was right. Things were different now.” This last part really made me want to think about the events that may have followed this meeting.
Well done on such great work!

That was a really fascinating take on Luna's marriage! You could spin a much bigger story from this, because I am already wondering how Luna and Rolf met, and why she disappeared, and more importantly, what happens next: does Harry go after Rolf? Do they reconcile with Luna? Why was Harry so angry?
I also loved seeing some music in the Potterverse! The way Luna played the piano fit perfectly with her character. Nice job!
~Gina :)