Here is the straight dope from IslamQA.com:
Can a muslim celebrate a non muslim holiday like Thanksgiving?
Published Date: 1998-03-29
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. . .”
“. . . This day, I have perfected your religion for you, completed My favour upon you, and have chosen for you Islaam as your religion . . .”
So congratulating them is forbidden, whether they are one’s colleagues at work or otherwise.
If they greet us on the occasion of their festivals, we should not respond, because these are not our festivals, and because they are not festivals which are acceptable to Allaah. These festivals are innovations in their religions, and even those which may have been prescribed formerly have been abrogated by the religion of Islaam, with which Allaah sent Muhammad (peace and blessings of Allaah be upon him) to the whole of mankind. Allaah says (interpretation of the meaning):
“Whoever seeks a religion other than Islaam, it will never be accepted of him, and in the Hereafter he will be one of the losers.” [Aal ‘Imraan 3:85]…
Asad Shah was murdered after he wished Christians a “Good Friday and very happy Easter, especially to my beloved Christian nation.”
“Scotland police probe killing of Muslim shopkeeper who posted Facebook Easter message to Christians,” FoxNews.com, March 26, 2016:
A popular shopkeeper who wished Christians a happy Easter on Facebook was stabbed to death in what police in Scotland say was a “religiously prejudiced” attack carried out by a fellow Muslim, it was reported Saturday.
A vigil was held Friday night in Glasgow’s Shawlands neighborhood in memory of Asad Shah who was killed the day before– a few hours after he apparently posted messages on Facebook that said “Good Friday and very happy Easter, especially to my beloved Christian nation.”
“Let’s follow the real footstep of beloved holy Jesus Christ and get the real success in both worlds,” one of the messages said.
Police said a 32-year-old man has been arrested in connection with Shah’s death. Police said the suspect is Muslim. He was not been identified.
Scottish First Minister Nicola Sturgeon joined the vigil in support of Shah and his family.
“Moved to be one of hundreds tonight as Shawlands united in grief for Asad Shah and support for his family,” Sturgeon tweeted afterward, the BBC reported Saturday.
According to the BBC, those attending the memorial were encouraged to bring a daffodil. Those at the gathering laid flowers and lit candles.
“It was very respectful,” vigil organizer Eildon Dyer told the BBC. “There were a lot of people clearly very upset. There were a lot of tears and lots and lots of flowers.”
Dyer added: “Everybody has said he was the nicest man. He was clearly much-loved. Everybody had nice stories to tell about him and warm stories. It’s just very, very sad.”…