Four-foot-tall Harry Potters, Waldos and Cruella de Vils could be seen all over the United Kingdom on Thursday as children celebrated World Book Day. All secondary schools in the U.K. are required to register to participate in the 18th annual charity event meant to encourage a love of reading, and this means thousands of kids can wear costumes to class.

World Book Day, which is planned in partnership with UNESCO, takes place on different days depending on the country. The U.K. recognizes it on the first Thursday in March, but most of the 100 nations that participate hold the holiday on April 23. Teachers host activities for World Book Day in class, like video chatting with authors, reading aloud and writing stories of their own.

"It’s all about getting kids closer to the books and authors they already love, and letting them discover more books and authors they’ll love every bit as much in the future," according to

In the U.K., organizers send vouchers redeemable at book stores to more than 14 million youth. This year's discounted books include but are not limited to "Elmer's Parade," "The Dinosaur That Pooped A Lot!", "Best Mates," "Goth Girl and the Pirate Queen" and "The Diary of Dennis the Menace." Many parents took these novels as inspiration for their kids' costumes. Other families relied on more traditional characters like the Grinch and Red Riding Hood.

World Book Day gave students and parents a chance to show off their DIY skills, as most costumes were homemade and involved makeup, wigs and, of course, a chapter of a book. Some were cute, others creepy, but all were creative. Even some teachers dressed up.

Age aside, thousands of people posted pictures of their World Book Day costumes on social media. See a few of our favorites below.