I see many "Muslims" joining in Christmas and other celebrations. Is there any daleel from the Quran and Sunnah that I can present to them to show that these are indeed very sinful practices?.
It is not permissible to join in the kaafir festivals for the following reasons:
Firstly: because this entails imitating or resembling them, and “Whoever imitates a people is one of them.” (Narrated by Abu Dawood). This is a serious warning. ‘Abd-Allaah ibn al-‘Aas said: Whoever lives in the land of the mushrikeen and celebrates their Nawrooz (New Year) and their Mahrajaan (festivals), and imitates them until he dies, he will be a loser on the Day of Resurrection.
Secondly: taking part in their festivals is a kind of befriending them and showing love for them. Allaah says (interpretation of the meaning):
“O you who believe! Take not the Jews and the Christians as Awliyaa’ (friends, protectors, helpers), they are but Awliyaa’ of each other. And if any amongst you takes them (as Awliyaa’), then surely, he is one of them… “[al-Maa’idah 5:51]
“O you who believe! Take not My enemies and your enemies (i.e. disbelievers and polytheists) as friends, showing affection towards them” [al-Mumtahanah 60:1]
Thirdly: festivals are the matter of religion and beliefs, not the matter of worldly customs, as is indicated by the hadeeth: “Every nation has its Eid, and this is our Eid.” Their Eid or festival reflects their corrupt beliefs of Kufr and Shirk.
Fourthly: “And those who do not witness falsehood, and if they pass by some evil play or evil talk, they pass it by with dignity” [al-Furqaan 25:72 – interpretation of the meaning]. The scholars interpreted this aayah was referring to the festivals of the mushrikeen. It is not permissible to give any of them cards for their festivals, or to sell them cards or any of the other things they need for their festivals such as lights, trees or food – including turkey, candy canes, etc.
We have already answered a similar question, for more details please see Question #947.