The follow statement is from a Boise Mosque
Can a muslim celebrate a non muslim holiday like Christmas?
Praise be to Allaah.Greeting the kuffaar on Christmas and other religious holidays of theirs is haraam, by consensus, as Ibn al-Qayyim, may Allaah have mercy on him, said in Ahkaam Ahl al-Dhimmah: "Congratulating the kuffaar on the rituals that belong only to them is haraam by consensus, as is congratulating them on their festivals and fasts by saying ‘A happy festival to you’ or ‘May you enjoy your festival,’ and so on. If the one who says this has been saved from kufr, it is still forbidden. It is like congratulating someone for prostrating to the cross, or even worse than that. It is as great a sin as congratulating someone for drinking wine, or murdering someone, or having illicit sexual relations, and so on. Many of those who have no respect for their religion fall into this error; they do not realize the offensiveness of their actions. Whoever congratulates a person for his disobedience or bid’ah or kufr exposes himself to the wrath and anger of Allaah."Congratulating the kuffaar on their religious festivals is haraam to the extent described by Ibn al-Qayyim because it implies that one accepts or approves of their rituals of kufr, even if one would not accept those things for oneself. But the Muslim should not aceept the rituals of kufr or congratulate anyone else for them, because Allaah does not accept any of that at all, as He says (interpretation of the meaning):"If you disbelieve, then verily, Allaah is not in need of you, He likes not disbelief for His slaves. And if you are grateful (by being believers), He is pleased therewith for you. . ."[al-Zumar 39:7]". . . This day, I have perfected your religion for you, completed My favour upon you, and have chosen for you Islaam as your religion . . ." [al-Maa’idah 5:3]So congratulating them is forbidden, whether they are one’s colleagues at work or otherwise.
http://youtu.be/L-VFK-CGLp8
A kafir is someone who rejects the teaching of Muhammad.