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

Other days

frozen yogurt

Frozen Yogurt Day
tell a fairytale

Tell A Fairytale Day
buddy

Buddy Day
bounce

Bounce Day
lisp

Lisp Day
eat what you want

Eat What You Want Day
martini

Martini Day
gum

Gum Day
offer

Offer Day
one

One Day