The full eNotes chapter summary set for Every Day is included in the link below. Meanwhile, I am happy to give a general summary here. To understand the story, the reader must understand the main character of "A." A is a kind of "spirit," neither male nor female, destined to inhabit different bodies for exactly 24 hours before moving on to the next one. A has to live in that particular body for the day, experiencing life in this way and sharing that person's memories.
The story begins when A awakens in the body of a teenager named Justin. When A (through Justin) meets Justin's girlfriend, Rhiannon, a problem arises: A, not Justin, becomes enamored with Rhiannon. A immediately notices that Justin and Rhiannon are having a tough time in their relationship. This presents a good opportunity for A. A (still inhabiting Justin's body) then invites Rhiannon to the beach where they share some very special time together.
A decides to stay with Rhiannon no matter what body he inhabits. First, A has to convince Rhiannon that he is not Justin. Every day, then, A has to figure out how to approach Rhiannon no matter what body he is in. (It is just luck that all of A's bodies seem to be within miles of each other.) Rhiannon finally believes A, but admits that she cannot have a relationship like this. Meanwhile, A has to deal with other characters he inhabits. A eventually realizes that he can never have a normal relationship with Rhiannon. However, A decides he always wants to be close to her. At the end of the book we find A inhabiting the body of Katie, who goes on a trip and tries to "disappear for good."