If you engage in hate, enable the hateful, or remain silent in the face of hatred, you cannot say "Merry Christmas".
If you have diminished your fellow man because of the color of their skin, you cannot say "Merry Christmas".
If you rejected people because of their faith in God, Allah, you cannot say "Merry Christmas".
For you, it's a holiday, because you're simply off from work.
Christmas is a celebration of the birth of Jesus Christ. Peace be upon him.
Merry Christmas. :)