Is it permissible to attend the celebrations of Christian festivals and congratulate them?
Ibn al-Qayyim (may Allaah have mercy on him) said: it is not permissible for the Muslims to attend the festivals of the mushrikeen, according to the consensus of the scholars whose words carry weight. The fuqaha’ who follow the four schools of thought have stated this clearly in their books… Al-Bayhaqi narrated with a saheeh isnaad from ‘Umar ibn al-Khattaab that he said: “Do not enter upon the mushrikeen in their churches on the day of their festival, for divine wrath is descending upon them.” And ‘Umar also said: “Avoid the enemies of Allaah on their festivals.” Al-Bayhaqi narrated with a jayyid isnaad from ‘Abd-Allaah ibn ‘Amr that he said: “Whoever settles in the land of the non-Arabs and celebrates their new year and festival and imitates them until he dies in that state, will be gathered with them on the Day of Resurrection.” (Ahkaam Ahl al-Dhimmah, 1/723-724).
With regard to congratulating them on their festivals, this has already been answered in Question no. 947, which we advise you to refer to.