You’re about to feel very old. September 1, 2017, marks the date that Harry Potter's son, Albus Severus Potter, boarded the Hogwarts Express at King's Cross for the first time.

We’re heading Back to Hogwarts with the BT TV Store to mark the occasion.

[Read more: The Harry Potter Bucket List - 9 places every fan should visit]

To celebrate the anniversary, the BT TV Store has a special sale so you can relive every magical moment from the JK Rowling series.

All eight Harry Potter films are available to buy in the BT TV Store now for under £10 each.

Back to Hogwarts: Harry Potter

We were shocked by Harry’s landmark date, but here are 10 more facts your probably didn’t know about the boy wizard.

1. Why weren't Harry's eyes green?

In the books Harry’s eyes were described as “bright green”, but there was a slight problem that Daniel Radcliffe’s are blue.
Green contacts caused an allergic reaction for Radcliffe and the post-production change would have been costly and time-consuming.

Luckily for Radcliffe, JK Rowling said the colour wasn’t important and the only important thing was they they looked like his mother’s eyes – so Lily Potter needed to resemble Radcliffe.

2. Rik Mayall was supposed to be in the films

The late great comic was cast, filmed and paid for his scenes as Peeves in the Sorcerer’s Stone, but he was never seen on the big screen.

Speaking about the experience in an interview in 2011, Mayall said:

“I got sent off the set because every time I tried to do a bit of acting, all the lads who were playing the school kids kept getting the giggles, they kept corpsing, so they threw me off.

“They asked me to do it with my back to them and they still laughed. So they asked me to do it around the other side of the cathedral and shout my lines, but they still laughed so they said they’d do my lines with someone else. So then I did a little bit of filming, then I went home and I got the money. That’s significant.

“Then a month later, they said: ‘Er, Rik, we’re sorry about this, but you’re not in the film. We’ve cut you out of the film.’ But I still got the money. So that is the most exciting film I’ve ever been in, because I got the oodle and I wasn’t in it. Fantastic.”

3. Rupert Grint didn't do his homework

Back to Hogwarts: Harry Potter 19 years later

Director Alfonso Cuarón asked the three young stars to al write essays about their characters for him. Radcliffe and Emma Watson responded with one-page and 10-page replies, respectively.

However, Rupert Grint didn’t bother handing in his homework.

“I didn't do mine because I didn't think Ron would. Or that was my excuse. At the time, I was actually quite busy with the real schoolwork involved with my exams, and I just didn't do it,” Grint told Entertainment Weekly. “But in the end, it felt right because that's what Ron would have done.”

4. JK Rowling turned down Michael Jackson

Harry Potter has inspired various spin-offs on stage and screen and theme park rides in the UK and America. However, one venture the author snubbed came from the late King of Pop.

Rowling revealed in an interview with Oprah that she turned down MJ’s offer to create a Harry Potter musical.

5. Harry's birthday is significant

Harry’s birthday, July 31, is also JK Rowling’s birthday. However, it was Hermione who was largely based on Rowling’s youth and she picked Hermione’s patronus based on the fact her own favourite animal is the otter.

6. The Dementors are personal to JK Rowling

The foulest creatures that walk this earth were based on Rowling’s personal demons and experiences with depression. She created the creatures following the death of her mother and they represented the dark feelings that depression can trigger in you.

7. Tom Felton could have been Harry or Ron

Tom Felton auditioned for the parts of Harry and Ron, but eventually landed a role as the dastardly Draco. Tom didn’t feel any bitterness to his co-stars, claiming that there was no doubt in his mind nobody could have played the roles better than Rupert and Dan.

Harry Potter: Back to Hogwarts

8. Hugh Grant was originally cast in the film

The bumbling British rom-com king was supposed to play Gilderoy Lockhart, but had to pull out because of scheduling conflicts. Jude Law was also apparently considered for the role but eventually Kenneth Branagh stepped in and stole the show.

9. Emma Watson nearly missed out

Casting for Hermione took place in schools around the country, but Watson had no desire to audition. It was only the persuasion of one teacher that got her to step up as the last girl of the day and catch the attention of the producers.

10. Dumbledore comes from the Old English word for bumblebee

JK Rowling picked the word because she always pictured Dumbledore humming to himself.

The Back to Hogwarts sale is available now in the BT TV Store and runs until September 10.

Press the BT Player button on your remote control, scroll across to Film and browse now

[Read more: Which Harry Potter book are you?]