Leap Day is a pretty much a by-the-numbers story of a girl turning 16 and what she does on her birthday. Each chapter covers about two hours out of her day. And the adventures (crushes on older boy, getting her driver's license, worrying about her body type, etc.) are fairly typical YA material. What makes this story different isn't that Josie's birthday falls on February 29, but what goes on in alternating chapters. Between each chapter (told first person) from Josie's point of view, is a chapter where all the characters she interacts with get to give their view of what happened. Thus, we get to read not only what Josie thinks people think, but what they really are thinking, and what they think about her.
This is an interesting and original literary device. It is also an interesting tool for observing how we tend to misconstrue other people's thoughts. And it also reveals a bit about Josie as we see how she allows her own thoughts and fears to prevent her from seeing what other people are thinking.
In many ways, though, this is a book that works better with older readers than its target audience. Mass takes some of these observations of others far into the future. A character might comment that he didn't ask Josie out, but that ten years from now he will when they run into each other at a reunion. I'm not sure how many teen readers will find that interesting. As an adult reader, I found it fun and fascinating. Sort of like Mass was playing God a bit.