Narrator and protagonist Rachel describes Darcy Rhone, her best friend since childhood. The book’s narration is in first person, told from Rachel’s point of view. Rachel has always felt that Darcy has been superior to her. When they were in fifth grade, they found a calendar at the back of a phone book that allowed them to determine the day of the week their birthday would fall in the future. Rachel envisioned her 30th birthday, which would be on a Sunday, imagining a dinner date with her future husband. Darcy, whose 30th birthday would be on a Monday, dismissed the concept of birthdays as being unimportant by the time they reached that age.

Rachel thinks about being 30 again a few more times before the actual birthday arrives, which she reveals is tomorrow. She attends the surprise party Darcy throws for her and feels some anxiety that her twenties are about to end. Her biggest desires are to have a job she’s happy with and a guy she loves, but so far, she has neither. While Rachel is an attorney at a large law firm in New York, the job isn’t what she hoped it would be.