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.
Read More... 5. Southey Junior School Staff and children at Southey Junior School in fancy dress as characters from books to celebrate World Book Day in 2001 Photo: DEAN ATKINS
6. Shortbrook Primary Marie Shaw from Shortbrook Primary getting in some bedtime reading with pupils Courtnay Lee and Adrianne Boulton on World Book Day in 2002 Photo: Dean Atkins
7. St Catherine's School Staff and pupils at St Catherines School Pitsmoor dressed up as their favourite characters for World Book Day in 2004 Photo: Dean Atkins
8. Southey Junior School Staff and children at Southey junior School in fancy dress as characters from books to celebrate World Book Day Photo: DEAN ATKINS