I think authors and illustrators hide some of their best work in endpapers (the Knuffle Bunny series, in which the endpapers show the parents' wedding photos and the seminal moment of each book in the series, comes immediately to mind, but there are lots of others) and Yes Day! is a perfect example. The endpapers are designed like a calendar, with a column for each day of the week. Each normal day is, of course, No Day. But in a clever stroke, each box shows a different way of saying no: "not gonna happen day," "we'll see day," "and that's final day," and "hands on hips day," among others.
I like to think that Yes Day! was not just inspired by a child who was tired of hearing no, but written by a mom who was tired of saying it. I know sometimes I am.