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A lot of leniencies have been given over the years, with the goal of not causing ill-will among non-Jewish co-workers, and improving work relationships. (It's quite amazing how many of the prohibitions in the first chapter of Avoda Zara are circumvented in one way or another by the Tosafists, which led one thinker to pen a monograph entitled "Was Rabbeinu Tam a Reform Rabbi?").

In an OU talk several years ago regarding various workplace issues, Rabbi Herschel Schachter insisted that saying "Season's Greetings" to a non-Jewish co-worker was permissible (or certainly "Happy New Year", I presume), but not "Merry Christmas."