So whether the costume was made or bought all that matters is that everyone has fun.
Take a look back at these great pictures spanning 21 years and see who you can spot.
1. Anne Grove Primary School
Parents at Anne Grove Primary School Heeley , Sharon Batty, Andy Coish And Androula Philippou read to some of the pupils at the school on World Book Day in 1999 Photo: Barry Richardson
2. Park Primary
Park Primary School pupils, from left, Gareth Hunter, aged nine, Ashley De-Viell, aged eight, Roxanne Jones and Rebecca Green, both aged nine, chose a book to read in 2000 Photo: Steve Taylor
3. Abbeydale Grange school
Author Denuta Reah at Abbeydale Grange school presented a cheque to the school library as part of World Book Day 2000, pupil Lisa Nettleship received it on behalf of the school Photo: Stuart Hastings
4. Sheffield High School
Sheffield High School pupils being talked to by author Anne Fine as part of World Book Day. Anne, centre with one of her books the Flour Babies, and Celia Lacey, left and Melissa Gibbons in 2001. Photo: Andrew Partridge (Staff)