One Day (Vintage Contemporaries) by David Nicholls

It’s 1988 and Dexter Mayhew and Emma Morley have only just met. But after only one day together, they cannot stop thinking about one another. Over twenty years, snapshots of that relationship are revealed on the same day—July 15th—of each year. They face squabbles and fights, hopes and missed opportunities, laughter and tears. Dex and Em must come to grips with the nature of love and life itself. As the years go by,  the true meaning of this one crucial day is revealed.