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?
Although, with that said, the prom seems to start at 6:00pm, and a lot of romantic activity tends to happen in the evening, or later. It seems unlikely her dad would want to talk business that late, so unless Aya had plans to spend the whole day with Koga, it realistically shouldn't cause that much of a scheduling conflict.
Still awful timing and a bit rude.