Final answer:
When Marian discovers it's Abbie's birthday, she becomes happier and more excited.
Step-by-step explanation:
The shift in Marian's mood when she discovers it's Abbie's birthday is that she becomes.This is evident in the text when Marian's mood is described as blissful and she is unprepared for any clash when Marjorie adjusts her hat in the mirror. This shift indicates that Marian is happy and looking forward to celebrating Abbie's birthday.