Congress recognized the Christian holiday. They didn't make up a secular holiday and decide to also call it Christmas.
I understand this, but I suppose that was lost in my post.
I mean to say that even if the atheists and ACLU objected to the term Christmas, they would have to challenge the federal law making it a federal holiday. To refuse to say "Merry Christmas" to folks because you object to the religious aspect ignores the fact that it is also a non-denominational, government-declared holiday, which makes it, de facto and de jure, a secular holiday we consider a HOLY day.