May you have a Blessed-Whatever-You-Celebrate. While I won't be the least bit offended, even by "X-Mas"- my apologies, extremists, but it's simply Greek--- (From ɛksməs- X-stands for Chi, the first letter of Christ in Greek) I understand the warmth that comes from being addressed with a cheerful greeting that doesn't wreak of the generic "Happy Holidays."
So, without further ado:
If you believe in no deity, whatsoever, Happy Nothing, or Happy Whatever, depending on whether your glass is half-full or half-empty.
If you're an ancient Mayan. Dude. That Apocalypse thing. Ouch. Congrats on time travel though. Happy New Age.
To everyone else: I wish you a warm, safe, and blessed Happy Christmahanukwanzasolsticekuh™ (All rights reserved :-P) a blessed Eid, a joyous Yule, God jul, Feliz Navidad. Na zadrovie!-To Your Health.
I am now breathless.