This is not your conventional love story. We follow the lives our two protagonists, Emma and Dexter, for twenty years. Once a year, on July 15 we check in on them and find out how they have been getting on with their lives. Nerdy Emma and carefree Dexter are polar opposites and yet beneath the surface there is a bond that keeps them together. Through little incidents we learn that Dexter is not as selfish as he appears to be and Emma is more generous emotionally than we expect. As time progresses we watch Emma and Dexter grow as individuals. We become involved in their lives. And finally Dexter admits to himself, what we, the readers have known all along that he is in love with Emma. What we discover with Emma and Dexter is that each perfectly fills the "hole" that is missing in the other.
There are many superstitions about St Swithin's Day and it isn't until the last few pages that we learn why Nicholls chose this day for his story. Would we have liked a different ending? Probably. But if we look at "One Day" as Dexter's story and his journey from a boy to a man then the ending is appropriate.