I wrote on this topic last year, but I've gotten some new readers since then, and in any case I feel strongly enough about the subject I think it bears revisiting.
Putting aside for the moment the difference between Advent and Christmas, there are those evangelicals and members of the religious right who protest the use of the term "Holidays," rather than "Christmas," and the use of "Happy Holidays" instead of "Merry Christmas." However, in spite of the efforts of the "W" administration and others to make make Christianity the state religion, the First Amendment does still stand: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof."
The fact is, there are a lot of people who are not Christians and who do not celebrate Christmas. My first wife was Jewish, and I'm still in touch with some of her family. A good friend and former co-worker of mine is a devout pagan. Terry and I deliberately select Christmas cards that do not use the word Christmas, but rather say things like "Peace on Earth." And I'm proud of it. Just because I choose to follow a Christian path doesn't mean I want to convert others to that path.
I do say "Happy Holidays!" and I'm proud of that too.