To start with, as far as I am aware, neither the city, the county, or the state, or the federal government has outlawed a citizen from celebrating Christmas in whatever manner they choose fit on their own property. What governments can and have done under the first amendment to the constitution is outlaw public funds being used to celebrate particular religious holidays. In other words the government may not use your tax dollars to celebrate a particular religion. For example, the government may not use your tax dollars to promote or celebrate Hindu, Jewish, Christian or any other religious holiday. This protects you from having to support, through your tax dollars, Hindu, Christian, Jewish or any other religion.
Now, how to greet people with good spirit during this holiday season.
If the person you are greeting is known to be a Christian, wish them a “Merry Christmas”. If you don’t know the person’s religion, but you still want to wish them well in this season, a generic “Happy Holiday” would be nice and appropriate.
Finally, for those who still want to present themselves as victims of a “War on Christmas”, to them I say “Happy Holidays” and a “Bah Humbug”!