To celebrate the 20th anniversary of Harry Potter and the Prisoner of Azkaban in the UK, Bloomsbury are creating a huge mosaic of Harry casting his stag Patronus – created out of your happy memories. Here’s how it works.

Bloomsbury, in partnership with Warner Bros. Studio Tour London, are asking Wizarding World fans around the world to post their happiest memories to Instagram or Twitter using the hashtag #ExpectoPatronum20 – in honour of the upcoming 20th UK anniversary of the third Harry Potter book, which first introduced us to the magic of the Patronus.

Yes, it’s been nearly two decades since we first met Remus Lupin, the Knight Bus, Sirius Black, Crookshanks and the true identity of Scabbers – 8 July marks 20 years since Prisoner of Azkaban was first published in the UK. The third book is also famous for introducing us to the Patronus charm, a particularly tricky spell where the wizard or witch must summon a happy thought or memory in order to conjure a ghostly, silver animal guardian to protect them. You can learn more about Patronuses here.

To mark the 20 years moment in style, Bloomsbury are asking you to post a photo to Instagram or Twitter of an especially happy memory with the hashtag #ExpectoPatronum20. This could be anything: maybe it’s when you first discovered Harry Potter. Maybe it’s when you got married. Maybe it’s when you had a particularly delicious sandwich. Whatever leaps to the forefront of your mind if you were to conjure a Patronus, simply post a photo (publicly, please) on Instagram or Twitter with the hashtag #ExpectoPatronum20, and your image might just be printed as tiles to create a huge mosaic. That mosaic will be displayed at Warner Bros. Studio Tour London on the big anniversary day.

This huge montage will show Jim Kay’s amazing illustration of Harry casting his Patronus, which takes the form of a stag, like his father’s. If you’re lucky enough to be visiting the Studio Tour on 8 July you can post your happy memory before you visit or on the day, and you’ll get to see the mosaic being created in real-time. But for anyone not visiting - you can still be part of this amazing experience and have the chance for your happy memory to be merged with thousands of others to commemorate this anniversary online. For more information on how the rules work, check out all the details here.

And remember the more happy memories we can gather, the stronger the Patronus gets – so be sure to tell your mates about it too. All together now, #ExpectoPatronum20.