"Hullo, Dad" (42.69). It's a simple last line, but Lily's decision to talk to the father she's been so angry at and estranged from proves her tremendous growth from the beginning of the story to the end. A key part of Lily's journey in One Whole and Perfect Day is learning to not only love her family as they are, but forgive them for their wrongs. And the way Clarke chooses to reveal just how far Lily's come in this regard this is a particularly symbolic and powerful moment in the story.
Lily's dad is a pretty key figure in her life, and his abandonment of their family has dealt her a pretty serious wound. We know this because she mentions him in the second paragraph of the whole book: "She'd never even seen her father, and when his phone messages came at Christmas and birthdays, she found she didn't know what to call him: 'Dad' sounded awkward in her mouth" (1.2). We're even told that one time, he actually called her "Lolly" instead of Lily (2.21). Oops. Big time.
In the end, though, Lily not only chooses to forgive her father, but does so in a particularly meaningful way: She casually calls him "Dad" for the first time in her life. If that's not a huge expression of the book's call to accept and love our families, we don't know what is.