Oops, it doesn't look like 'smack a bum day' is a national day..

Other days

ride the wind

Ride The Wind Day
nail polish

Nail Polish Day
hoodie

Hoodie Day
tell a fairytale

Tell A Fairytale Day
gum

Gum Day
kale

Kale Day
frozen yogurt

Frozen Yogurt Day
camera

Camera Day
ruby

Ruby Day
lisp

Lisp Day