It's just being nice, if you get offended when someone is wishing you a happy time then maybe you should rearrange your priorities and get pissed at more important problems.
I know some Jehovah witnesses they don't celebrate anything. And they just say thanks or nothing, but don't argue. I think most people are ok. Just the few confused Christians. I am of Christian belief myself and if u say happy Hanukkah to me I will be ok with that, you mean well. It's not like Christ was actually born on a December 25th anyways. We celebrate it that day. And if u believe every other religion is wrong, let them wrongly celebrate and be happy!
A celebration around winter solstice has been a tradition in (northern) europe way, way before christianity. And the term "holiday" literally means "holy day". So any "christian" complaining about hearing "happy holiday" is a dork who is trying hard to be offended by someone wishing him/her a good time celebrating whatever they want to celebrate.
The only time I get pissed off is when I'm wearing a Magen David (Star of David) or I'm talking about Hanukkah in front of the clerk and they still wish me Merry Christmas.
I don't know anybody who celebrates christmas as a christian holiday. Before it was christian it was pagan anyway, and any connection to religion was put aside nowadays. Appart from a few christians, almost everybody in France celebrate christmas as a normal holiday.
Isn't this all so trivial. I don't believe in any of this nonsense but there's no point causing problems unless someone is actively trying to convert me or is preaching hate in the name of their magical entity. Incidentally, if you are offended by my comments here then don't worry, I'll burn in hell equivalent. Or be reincarnated as a slug.
"Merry Christmas"
"Happy Kwanza"
And both go about their lives without raising a stink.