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

Other days

bow tie

Bow Tie Day
comfy

Comfy Day
unicorn

Unicorn Day
martini

Martini Day
blanket fort

Blanket Fort Day
slime

Slime Day
boy friend

Boy Friend Day
hiv awareness

Hiv Awareness Day
cat girl

Cat Girl Day
bonang

Bonang Day