It is a day to celebrate books and reading and school children typically dress up as their favourite book character.
Have a look at some of the children in Derry dressed up today.
1. Harry Potter and the Little Princess
Niamh as the Little Princess and Aaron as Harry Potter.
2. Matilda
Allie as Miss Trunchbull from Matilda
3. Matilda and Amanda Thripp
Matilda and Amanda Thripp ready for the day.
4. Matilda
Cael, age 10, as Bruce Bogtrotter from Matilda