It’s only happened once in my life, but I can recall the moment I was verbally assaulted by a customer for responding to an offered “Merry Christmas” with “Happy Holidays.” The crystalline clarity of that exchange has far outlasted most other memories from that early job (grocery store bagging clerk).
It didn’t matter that “Happy Holidays” was the corporate-approved response that we were required to say; I’d have said it regardless, because I’m just that sort of person. I don’t like doing things just because they’re tradition. I like doing them because I like doing them.
Consequently, I also like conducting myself in such a way as to communicate my personal belief that the world does have more than one religion. I think there’s, like, four, based on holidays: Christmas for Christian people, Hanukkah for Jewish people, Winter Solstice for Pagan people, and Kwanzaa for . . . I don’t know what religion celebrates Kwanzaa. Seems a little racist to just say “black people,” you know?
I guess there’s also Boxing Day for Canadian people but finding out the truth about Boxing Day was a big disappointment because I used to imagine there was a holiday dedicated to fist fighting and that made me happy.
However, I do rather like my current job and would prefer to avoid creating irritated customers, which is why I have embraced my current holiday response with enthusiasm. It’s basically a form of verbal Jiu Jitsu, in which you attempt to trap your opponent in a position that they cannot retaliate from. Here’s my brilliant technique:
Customer: “Thanks, and have a Merry Christmas!”
Me: “Likewise!”
Likewise is perfect. It’s unassailable. It tells the person offering their particular well wishing exactly what they want to hear, without actually endorsing any particular holiday. You’ve trapped them in a verbal situation in which they cannot reasonably respond with offense.
Try to imagine such a person getting offended by likewise:
Customer: “Thanks, and have a Merry Christmas!”
Me: “Likewise!”
Customer: “What, you can’t wish me a Merry Christmas? HAVE YOU NO SOUL?”
Me: “What exactly do you think ‘likewise’ means?”
Checkmate. They can’t wriggle out of the fact that you expressed the offered greeting back at them, but you also can’t be held accountable for supporting any particular religious tradition, especially if you work in a secular or corporate establishment where that sort of thing is not policy. If you think this sounds overwhelmingly cynical, you are clearly not a person whose affection for the holidays is lukewarm, at best.
I could probably write a book about this: Christmas for Cynical People. I’m sure it’d be a great stocking stuffer.
Cynical Idealism 
I looked up some facts about Kwanzaa because that’s what I do. Futurama-esque jokes aside, I really don’t know anything about Kwanzaa.
It turns out I’m wrong about any particular religion celebrating Kwanzaa; it’s not a religious holiday but a cultural one, created by Maulana Karenga in 1966 as the first specifically African-American holiday. Karenga’s goal was to “give Blacks an alternative to the existing holiday and give Blacks an opportunity to celebrate themselves and their history, rather than simply imitate the practice of the dominant society.”
Interesting. This does mean that my attempt at humor in the post has been undone by reality, as Kwanzaa is literally “celebrated by black people.” I thought about removing the paragraph, but since the joke is directed at my expense due to my own personal ignorance, I think it still works in context.
On the other hand, I learned something new about a cultural tradition of which I had previously been ignorant. Overall, I’ll call that a positive net gain.
I wished “Happy Holidays” to someone Monday, and she turned to correct me with a friendly but firm “No… It’s Merry Christmas.”
That’s when you bust out the ultimate retort: “Happy Saturnalia!”
I’d say “Likewise!” is an elegant solution, and should be used more.