The story follows Emma (Mod) and Dexter (Woodall) elliptically over almost two decades, dropping in on them once a year on the same date: July 15 or St Swithin’s Day. That’s the first day they spend together as students in 1988, which, for most of the story, seems to explain the date’s significance and the reason the novel keeps returning to it.
However, it’s not until the book’s Tess of the D’Urbervilles epigraph – read aloud in the Netflix adaptation’s finale – that the actual significance of July 15 is revealed. In a rule-breaking flashback to a different date, Em reads the quote to Dex in December 1988. She’s bought him Tess for Christmas, predicting that he’ll never read it, but wanting to share this one idea that blew her mind as a teenager:
“[Tess] philosophically noted dates as they came past in the revolution of the year. Her own birthday, and every other day individualized by incidents in which she had taken some share. She suddenly thought, one afternoon, that there was another date, of greater importance than all those; that of her own death; a day which lay sly and unseen among all the other days of the year, giving no sign or sound when she annually passed over it; but not the less surely there. When was it?”
Tess here is considering the date of her death – a dark mirror of her birthday that she lives through each year without knowing the significance it will one day take.
One Day finale spoilers below
In the final section of the book and the penultimate episode of the TV adaptation, Emma is killed when she’s knocked off her bicycle in a traffic accident. Not long after she and Dexter have finally found each other, they lose each other. It’s the cruel twist for which the novel became famous, but it’s more than just that. It’s also the point at which the story’s entire shape is explained. July 15 is the “sly and unseen” day of Emma’s death, “giving no sign or sound when she annually passed over it; but not the less surely there.”