Which Harry Potter novel was the first to sell more than one million copies in its first day in the United Kingdom?

Harry Potter and the Order of the Phoenix was the first Harry Potter novel to sell more than one million copies on its first day in the United Kingdom.

The fifth novel was published on 21 June 2003. UK sales passed one million copies during its first day, an extraordinary result for a children’s book and a sign of the series’ global reach. It was also substantially longer than the early novels, with Harry facing new restrictions at Hogwarts and the growing threat of Voldemort.

The book introduced Dolores Umbridge as Hogwarts’ High Inquisitor and included the formation of Dumbledore’s Army. Its title refers to the secret organization originally created to oppose Voldemort.

Later installments produced even larger launch records. Harry Potter and the Deathly Hallows, published in 2007, set major first-day sales records internationally, so the fifth book’s distinction concerns the first million-copy UK opening within the series.