Sal was in a hurry to get to Idaho because she wanted to make it in time for her mother's birthday, which was seven days away. Although Sal knew that her mother wasn't coming back, she still believed that reaching Idaho on time would make a difference. It would allow her to "reclaim" her mother.
In Sal's mind, her mother wasn't dead; she was just "away." Although Sal didn't know what she would find in Idaho, she was convinced that she could bring her mother back if she reached Idaho by a certain date.
The text tells us that Mrs. Cadaver confirmed the death of Sal's mother to her. However, Sal didn't want to believe it. She imagined that she would find her mother somehow, connect with her again, and bring her home.
Sal's denial is indicative of someone going through the stages of grief. It's only after she sees her mother's tombstone that she's finally able to admit the reality of her mother's death.