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

Other days

tulip

Tulip Day
lisp

Lisp Day
artists

Artists Day
healing

Healing Day
book

Book Day
hot chocolate

Hot Chocolate Day
tell a fairytale

Tell A Fairytale Day
my little pony

My Little Pony Day
nail polish

Nail Polish Day
comfy

Comfy Day