I see many "Muslims" joining in Christmas and other celebrations. Is there any evidence from the Quran and Sunnah that I can present to them to show that these are indeed very sinful practices?
Summary of answer
It is not permissible to join in the non-Muslim festivals because this entails imitating or resembling them and taking part in their festivals is a kind of befriending them and showing love for them.
Answer
Praise be to Allah, and blessings and peace be upon the Messenger of Allah:
It is not permissible to join in the non-Muslim 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 Dawud). This is a serious warning. `Abdullah ibn Al-`Aas (may Allah be pleased with him) said: Whoever lives in the land of the disbelievers and celebrates their Nowruz (New Year) and their Mahrajan (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. Allah says (interpretation of the meaning):
“O you who believe! Take not the Jews and the Christians as Awliya’ (friends, protectors, helpers), they are but Awliya’ of each other. And if any amongst you takes them (as Awliya’), then surely, he is one of them…” [Al-Ma’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 matters that are related to religion and beliefs and not to worldly customs, as is indicated by the Hadith: “Every nation has its Eid, and this is our `Eid.” Their `Eid or festival reflects their corrupt disbelief and polytheism.
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-Furqan 25:72 – interpretation of the meaning] The scholars interpreted this verse was referring to the festivals of the non-Muslims. 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.
And Allah knows best.
Source: Sheikh Muhammed Salih Al-Munajjid