Whether we're talking about Jay Gatsby, Harry Potter, or even Ariel from The Little Mermaid, every great protagonist has one thing in common: Each has something he or she wants desperately, and is willing to do whatever it takes to get it. Lily Samson is no exception. All she wants is one day without family conflict, hurt feelings and tears, Pop's ax and racism, and Mom's old people ruining everything. You might even call it… One Whole and Perfect Day. Oh wait—that's exactly what Clarke's called it.
Throughout the book, the question of whether normalcy is even possible for her family haunts Lily. Their failure to be a "proper family" (1.1) inspires emotions ranging from anger to worry to depression; she considers them "freakish" (13.11) and ultimately thinks that they're just "too dysfunctional" (35.49). And since we as readers get the full picture of what life is like for Lily's family as well as Lily, we can see that she really does kind of get the short end of the stick in this bunch. The title, then, is a shout-out to what Lily wants, as well as what she usually doesn't get.
On the other hand, though, Lily still has hope that her dream of a perfect day might come true—in fact, that it's something that everyone is entitled to at least once. "Why shouldn't their family have one brilliant, perfect day?" she asks herself. "Wasn't such a day something everyone had a right to, a day you could always remember, no matter what happened to you ever after in your life? A whole perfect day?" (27.36). We can't say whether Lily's right in saying this, but one thing's for sure: She gets her wish. And when she does, the book's title comes true.