Oops, it doesn't look like 'tell her no day' is a national day..

Other days

slime

Slime Day
girl friend

Girl Friend Day
brow

Brow Day
cat girl

Cat Girl Day
boba

Boba Day
boyfriend

Boyfriend Day
blanket fort

Blanket Fort Day
offer

Offer Day
ormille

Ormille Day
china

China Day