It is reported that ʿUmar b. Al-Khattāb – Allāh be pleased with him – said, “Stay well away from the enemies of Allāh during their festivals.”
He is also reported to have said, “Do not learn the speech of the non-Arabs (unbelievers) and do not enter upon the polytheists in their churches during their holidays, for the wrath [of Allāh] descends upon them then.”
It is also reported that ʿAbdullāh b. ‘Amr b. Al-‘Āṣ – Allāh be pleased with him – said, “Whoever takes residence in the lands of the non-Arabs (unbelievers) and takes part in their Nayrūz and their Mahrijān (two Persian festivals) and copies them until the day he dies will be resurrected and gathered with them on the Day of Resurrection.”
Al-Bayhaqī, Al-Sunan Al-Kubrā Vol. 9 p234. Ibn Taymīyah (see reference below) states that the chain of transmission for the second statement of ʿUmar is ṣaḥīḥ.
Commenting on these narrations, Shaykh Al-Islām Ibn Taymīyah (Majmū’ Al-Fatāwā Vol. 23 p325) states, “So ʿUmar forbade the learning of their language and merely entering upon them in their churches on their festivals, so what about one who does what they do? Or what about someone who wants to do something that is part of their religion? Isn’t agreeing with them in actions even more serious than agreeing with them in their language? Isn’t doing some of the things they do in their festivals even more serious than just entering upon them during their festivals? And if the wrath [of Allāh] descends upon them during their festivals because of the things they do, isn’t the person who partakes in their actions or some of their actions exposing himself to the punishment for this? And the statement of [ʿUmar], ‘Stay well away from the enemies of Allāh during their festivals,’ isn’t this a prohibition from meeting with them and getting together with them? So what about a person who actually does what they do?”