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

Other days

bounce

Bounce Day
boyfriend

Boyfriend Day
tell a fairytale

Tell A Fairytale Day
hannah

Hannah Day
brow

Brow Day
offer

Offer Day
eat what you want

Eat What You Want Day
pumpkin

Pumpkin Day
ruby

Ruby Day
stud

Stud Day