The rehearsal script for Harry Potter and the Cursed Child (CC) is published worldwide at midnight on July 31st, 2016. Statistics show it to be the most pre-ordered book since Harry Potter and the Deathly Hallows (DH) back in 2009.
Just like for the release dates of the last several Harry Potter novels, many bookstores, libraries and other organizations held midnight release parties so that people who pre-ordered can be the first to get their hands on a copy.
The final version of the playscript was published a year later.
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 30 July 2016: Harry Potter and the Cursed Child, Parts One and Two