Headcanon that one day, after Christine has returned from Erik’s home and after she saw his face, Christine gets a small note from the opera ghost (who she now knows is actually plainly named Erik). It simply asks if she’ll visit with him that afternoon, and that they can go on a carriage ride or stay in and enjoy music together if she should like.
Of course she doesn’t exactly want to go, but when she spots a pair of sad eyes watching her above the stage she decides to accept his invitation.
And so Christine meets Erik that afternoon in her dressing room and he’s much more talkative and almost giddy acting. After 20 minutes in his home, with Erik talking a million miles per hour and switching between trying to entertain her and showing off different objects in his home, she finally asks him bluntly why he asked her to visit today.
And then Erik gets really quiet and shy, and he won’t look at her only at the ground. Christine is afraid she has somehow upset him with her question, but then she hears him mumble, “Today is my birthday.”
He just wanted to have someone to talk to on his birthday, and there’s no one else he would rather see than her.