Which Harry Potter book was the first J.K. Rowling novel to sell more than one million copies in one day?

Harry Potter and the Deathly Hallows was the first J.K. Rowling novel to sell more than one million copies in one day in the United Kingdom. The final main Harry Potter book was released on 21 July 2007.

The launch became a major publishing event, with midnight sales, large bookstore crowds and coordinated international publication. In the United Kingdom, the book sold more than two million copies during its first 24 hours, surpassing the previous record held by Harry Potter and the Half-Blood Prince.

The detail is sometimes misremembered as a worldwide one-day figure. The commonly cited record refers specifically to United Kingdom sales. Deathly Hallows concludes Harry’s central confrontation with Lord Voldemort and was published ten years after the first novel.