Been a while since you played FFXIII? Never played the game? This section contains all of the game's Datalog entries on the game's story to refresh your memory. Spoilers ahead!
Final Fantasy XIII begins in medias res, or in the middle of things; there were actually thirteen days leading up to where the game begins. This is where most of the conflicts in the game took root.
The game itself is divided into thirteen chapters. (There's kind of a theme going here, you see.)