Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Whimsical
For a book about a dysfunctional family, there's a certain element of magic that shines through the Samsons' various predicaments. Stan may say that "Coincidences do happen" (34.78), but the sheer amount of reunions, changes of heart, and reversals of logic that happen in this story almost makes it seem like there's a supernatural force behind it all. Perhaps that's another reason for Clarke's selection of third-person omniscient narration—a bird's-eye view of the story gives it the feel that something larger than the characters is arranging all of this.
Lily's wish for a perfect party at Nan's carries a whimsical quality in itself: "She kept dreaming of Nan's garden, of flowers and streamers and fairy lights twinkling in the trees […] Wasn't such a day something everyone had a right to?" (27.36). What's even more incredible, though, is that it turns out better than she could have planned, so much so that she "couldn't get her head around it all" (42.1). Daniel asks her out, Sef turns out to be a real person, and Pop insists that Clara try on his mother's wedding dress. Against all odds, victory abounds.
The fact that Lily gets her wish is magical in itself, but the day is so full of surprises that it rises above her expectations. As she thinks to herself, "It was wonderful but weird all the same, the kind of miracle that happened in fairy tales and certainly never occurred in families like theirs" (42.7). When even the main character can't believe quite how well things work out, we know the tone of the book we're reading is whimsical. This book dares to imagine just how good life can be.