Sure. There could be. Writers are often inspired by real - life events and might be able to create a novel in a day if they are highly motivated and the story is already well - formed in their minds. It might not be a long or complex novel, but it can be based on a true story.
Definitely not. 'One Day' is purely fictional. The author likely used their creativity and storytelling skills to come up with the plot and characters, rather than basing it on actual events or people.