I have so many of these little ideas in my head. That one just needed to get out, though. I was thinking of the days Severus would celebrate, himself. Christmas he does for Alstromeria. The days he'd celebrate would be her birthday, Winter Solstice (I think... I always feel he's in tune with the cleansing power of winter and snow), and the day that she became his and his alone.
I don't think he's ever had anything that was so utterly precious that he didn't have to share before.