Question: A writer says, "It is the right of Christians to celebrate their religious festivals. Muslims, too, should share in the joy of Ahl al-Kitab, with whom they live in peace on the basis of mutual respect, and celebrate their religious festivals. I do not see any harm in this." Does it not take a Muslim out of the fold of Islam? In Islam, is it permissible to celebrate religious festivals of non-Muslims?
ANSWER
It is in no way permissible. The following is written in fiqh books:

"It is not permissible to give anything to anyone in honor of the days of Nowruz or Mihrgan, which are Zoroastrian holidays. It is haram to give presents in the name of these days or with the intention of observing these days. If one does so because one respects these days, one becomes a disbeliever [kafir] because these days are respected by polytheists. Abul Hafs al-Kabir states that if one worships Allahu ta’ala for 50 years and then gives an egg as a present to a polytheist in honor of the Day of Nowruz, one will become a disbeliever. However, if one gives a present to a Muslim on this day without paying a special attention to this day or because one has to follow the custom, one will not become a disbeliever. If one who bought on that day something which one would not buy on any other day did so because one respected that day, one would become a disbeliever. If one bought it only for consuming it without specially observing that day, one would not become a disbeliever" (Durr-ul-Mukhtar, Vol. 5, p. 481).

It is written in Bazzaziyya, "Nowruz is Magians’ day of feast. It is disbelief to join Magians and to imitate them on that day." A person who imitates disbelievers at Christmas, Easter, or on other feast day will become a disbeliever. (Se'adet-i Ebediyye)

"It will be kufr [disbelief] to wrap round the waist a special sash called zunnar worn by priests; to worship idols, statues, the cross, that is, two sticks intersecting each other at a right angle representing, according to Christians, Jesus' sacrifice; to wear a cross with reverence; to insult what we have been ordered to show reverence to; and to show reverence to what we have been ordered to despise. If one does any of them, one will become a disbeliever" (Sharh on Birghiwi's Wasiyyatnama, pp. 115 202).

Hadrat Imam-i Rabbani states:
"It is shirk [polytheism] to respect the festival days of Hindus [also Nowruz, Christmas, and Easter] and to imitate their customs on those days. It causes disbelief. On festival days of disbelievers, the ignorant ones of Muslims do as disbelievers do, think of those days to be Muslims’ festivals, and send presents to one another like disbelievers on those days. They ornament their furniture and meal tables as disbelievers do. They distinguish those nights from other nights. All these are polytheism and disbelief" (Maktubat, Vol. 3, Letter 41; Se'adet-i Ebediyye).

Hadrat Ibn ’Abidin states as follows in the fifth volume:
"It is permissible to greet a zimmi or to make musafaha [hand shaking] with him when it is needed to do so. It is not permissible to do so for reverence. Reverence to a disbeliever causes disbelief" (Se'adet-i Ebediyye).

"It is disbelief to respect a disbeliever, to greet him/her in reverence, or to address him/her as "my master" (Bariqa, İslam Ahlakı).

The writer that you mentioned in your question encourages Muslims to revere non-Muslims. It can be inferred from the excerpts above that such reverence is disbelief, too. It is not reasonable to fall into disbelief in order to please Christians.