Which Harry Potter book was the first in the series to sell more than one million copies in the UK in one day?

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

The fifth novel was published on 21 June 2003. Its launch generated extraordinary demand, with more than one million copies sold in the UK on the first day. This milestone reflected the series’ rapid growth from a successful children’s fantasy into a global publishing event.

The book is the longest novel in the original series in many English editions. It introduces the authoritarian Hogwarts administrator Dolores Umbridge as a major antagonist and follows Harry’s growing concern that the wizarding authorities are refusing to acknowledge Voldemort’s return.

The later books also broke sales records. Harry Potter and the Deathly Hallows, published in 2007, sold millions of copies worldwide during its first day, but Order of the Phoenix holds the notable UK one-day milestone asked about here.