Last time I checked there was more than one holiday in December. Hanukkah, Kwanzaa, Yule, Christmas, Boxing Day and New Year's Eve just off the top of my head. The word holiday comes from the notion of a "Holy Day". Every religion has holy days. I don't see how telling someone to have a happy holiday is offense. If anything people are upset because it's including non-Christians. "How dare people believe in something other that what I believe in!" The sentiment is sincere, whatever you celebrate this season, I hope it's joyful. How dare I!
Furthermore, historically Jesus Christ was not born on December 25th. More accurately he was probably born in the end of September during the annual Feast of Tabernacles. He may have been conceived in December though. The Romans decided to celebrate the Christ Mass to coincide with the Winter Solstice. The Church wanted to replace the Pagan festival with a Christian holy day.
Whatever reason we celebrate Christmas today, the season has become a time to spread cheer, joy and love. To hold your loved ones close and help those less fortunate. How wishing someone a Happy Holy Day is somehow going to cheapen that is beyond me. Christ, if you chose to believe in him, was the embodiment of all of those things and I'm sure he wouldn't be offended.
