Q: Some Muslims join with Christians in their feasts, what is your advice?
A: It is not permissible for Muslims to join Christians, Jews, or other Kafirs (disbelievers) in their feasts. In fact, it is obligatory on them to avoid doing so, because anyone who imitates a people is one of them. The Messenger (peace be upon him) warned us against imitating them or copying their morals. A believer should be careful of not doing so. Also it is not permissible for a believer to help the celebrants in this in any way, because their feasts conflict with our Shari'ah. So it is not permissible to join in celebrating them or even to cooperate with them or help them with anything, such as making tea, coffee, or providing them with utensils or anything else, because Allah (Exalted be He) says:
Help you one another in AlBirr and AtTaqwâ (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allâh. Verily, Allâh is Severe in punishment.