Dare one consult the U.S. Constitution? "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..." Celebrating Christmas in government offices is so far removed from what is prohibited that it is a joke.
"Christmas" in American society has evolved into a holiday that has almost nothing to do with celebrating the birth of Jesus Christ. One could argue that, aside from the name, the celebrations, customs, and rituals have very little to do with the religious celebration that falls on the same date.
Christmas tree? Forget about it. No connection.
Gift-giving? I don't think so. The gift giving in the Bible took place well after Christ's birth, so that's a modern invention (gift-giving at Christmas). In fact, in many Christian cultures the gift giving is on January 6th, the date when we remember the visit of the three Magi.
Santa Claus? Sort of a religious name, but the Santa Claus back-story has nothing to do with the birth of Christ or with Christianity.
"Merry Christmas"? Have a happy day when celebrating the holiday? Nothing religious about that.
If the President had balls (and if he were not a Muslim) he would simply send out a communication recognizing that our American society has created a separate secular holiday that we call Christmas, easily distinguishable from the religious celebration of the Nativity (which falls on the same day), and "we" are going to celebrate the Christmas HOLIDAY (as opposed to the Holy Day) with vigor and joy.
It would actually be a Teaching Moment for parents raising kids as Christians, emphasizing that BOTH celebrations need to be respected.