I don't get it. Why on earth would anyone want to have an ANTI-Halloween party?
Just have your own party where dressing is optional.
I'm too much of a Halloween freak to understand that, well that's not true, I get it, just can't fathom it.
How about a simple orange and black cake with simple writing:
Any resemblance to Halloween is purely coincidental.
You'd have to write really small...