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. Park Primary
Park Primary School pupils Nicola Cooke, aged 11, and Nomsa Masamvi, aged ten, take part in a World Book Day event at Wheatley Library in 2004 Photo: Steve Taylor
2. Sunnyfields
A whole caste of characters at Sunnyfields World Book Day in 2016 Photo: submit
3. Ahoy me hearties
The Balby Academy finance department dressed up as characters from Treasure Island for World Book Day 2014 Photo: Andrew Roe
4. Balby College
World Book Day , at Balby College where people dressed up as characters from Alice in Wonderland in 2013 Photo: Steve Parkin