One Day feel like a concept film, an experimental trial of style regardless of its impact on content. The movie starts with Emma (Hathaway) on July 15, mid-2000, then reverses back to July 15, 1998 and goes forward year by year until we catch up to the start of the film. The relevance of this day is obfuscated for no particular reason. It initially seems as if it commemorates the day she first meets Dex (Sturgess), as it follows their interactions on that day each year, but the true "secret" is revealed near the end. After that secret is revealed, it goes even further back to uncover even more unseen scenes.
And that is the real problem with the movie. It is mysterious for the sake of being mysterious. It does it to hide the story in the hopes of surprising you at the end, instead of doing it for any thematic benefit. You watch the first half of the film wondering why you're sitting there watching the film at all. This frustration is made worse by the fact that there is no chemistry between the two leads. There is no electricity or excitement. The acting is competent, but there is nothing tying the two together through time and space. They are two independently adequate actors. Ultimately, it doesn't matter how good the technical aspects are (they are decent) because the movie fails to engage us with its storytelling.
IMDb link: http://www.imdb.com/title/tt1563738/