I prefer: "Thank you so much for the offer - but I think it's best to actually follow Christ's commandment and not celebrate ignoring it. Toodles and have a nice day"
But these would be fun to:
"I would.......but the bread sucks and it doesn't really go well with dark wine"
"You know - it's hard to really get into a party when noone participates."
"Can I bring my own glass? I don't like sharing with strangers........"
"I really don't enjoy re-runs"
"I have plans at that time - how 'bout you stop over afterward and we can have our own celebration?"