Congratulating someone specifically for a rite of kufr — greeting them for the feast as a feast, in a way that signals approval of it — is forbidden, and Ibn al-Qayyim transmitted agreement among the scholars on this, because it amounts to being pleased with what Allah is not pleased with: "He does not approve of disbelief for His servants" (39:7). This is entirely separate from kindness and justice toward non-Muslims, which Islam commands: being a good neighbour and colleague, visiting them when ill, accepting lawful gifts, and dealing well with relatives who are not Muslim, for "Allah does not forbid you from those who do not fight you because of religion... from being righteous toward them and acting justly toward them" (60:8). Some contemporary scholars permit a neutral courtesy that endorses nothing of the creed, such as saying "enjoy your holiday" to a colleague, and this is a genuine difference of opinion among people of knowledge; but all agree that a Muslim may not attend or take part in the religious rite itself, for the servants of the Most Merciful are "those who do not witness falsehood" (25:72) — a verse that Mujāhid, Ibn Sīrīn and al-Ḍaḥḥāk explained as including the festivals of the idolaters.
Q&A · Rulings
What is the ruling on congratulating non-Muslims on Christmas and their religious festivals?
Evidence — the daleel
Quran 39:7Quran 60:8Quran 25:72Ibn al-Qayyim, Ahkam Ahl al-Dhimmah — chapter on the festivals of the People of the Book
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.