I know Christmas isn't as big of a thing in Japan, but inviting her over to, presumably, talk to her dad about music on that day still seems like a bit much. Even without the prom that was just announced, isn't it still something of an event where you'd expect people to be making plans with their families, friends, or loved ones?
I was thinking the same thing like wow girl of all days.....but then it got me thinking that, maybe, whatever exciting opportunity her dad will offer, she's planning to make it like a christmas gift for her? she does seem to like Mitsuki in an awestruck way, i can see her taking the chance to make it extra sweet if it's that time of the year anyhow...