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Saying Happy Holidays is not in the least bit offensive. Chances are, the person is celebrating SOMETHING this time of year. And even if they don't, they're still aware of the festivities that surround them and it still feels like a "holiday season" for them. At my job, we're told to say happy holidays rather than anything else as to not offend anyone. If they say Merry Christmas to you, it's more than okay to say it back.