Here be spoilers.
A wakes up every day in a different body. Ze gets the memories of the person ze is for that day and has to live like that as best as ze can. Usually, ze tries to disturb the life of the person as little as possible. But when ze wakes up in the body of Justin and meets Justin's girlfriend Rhiannon, ze breaks that rule.
And if you think that this goes on to be some soppy teenage love story, you would be wrong. It's not and it doesn't take the easy way out. The ending is hopefull, but not happy. And while A and Rhiannon are busy trying to figure out their feelings and how to make it all work, the books gets to explore quite a number of interesting things. How we perceive each other and how important appearance is, for one thing. How much can change in just one day.
By far the most interesting thing about Every Day for me was the fact that the people A lives in are not all heterosexual. Some are, others are gay or bi or trans and it's never treated as anything but normal. A hirself doesn't have a gender. Ze calls zirself Andrew sometimes, but ze sees hirself as neither or maybe both male and female. There is little enough queer represetation in young adult books and this is a very good example. The characters are not defined by their gender or sexuality, but it's there.
Levithan manages to introduce us to a new character in every chapter and to give them personality with just a couple of sentences. I found myself caring about every single one of them...something other authors don't manage with their protagonists. I read the German translation and I found the language a bit too adult for a 16 year old, but that may have well been the fault of the translation. And A is not really the typical teenager.
I'm very thankful that this is not the first part of a trilogy. The potential is there, but I hate the trilogy trend with the strength of a thousand suns and the book is much better ending where it does. Authors don't need to explain or explore every little thing about their characters.