The title of this book—The True Meaning of Smekday—works on two levels. The first is kind of obvious if you've read the book since it comes up on the first page, but since that's never stopped us from running our mouths before, we'll go ahead and state it: The title of the book is also the title of the essay assignment Gratuity is asked to write.
On a less obvious level, though, the title is also the question that Gratuity considers throughout the novel. In this sense it isn't so much about what Smekday is as what it is to her—what impact it's had on her life. So while at first glance, Smekday is the name the Boov gave Christmas once they invaded, the changes it represents go way beyond this. For Tip, Smekday isn't just about aliens or invisible guns; it's about the journey of a lifetime and saving the world. You can't get much better than that.