All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
A Muslim should be thankful to Allah for His great blessings upon him. It is he Who Guided him to the right path. So he should keep on supplicating Allah to make him steadfast on this path which is the path of blessed people and to keeping him away from the path of those who deserve His Anger and of those who went astray.
The non-Muslims spare no efforts to draw the Muslims away from the straight path and to cut their relation with their true religion. Allah Says (what means): {Many of the People of the Scripture wish they could turn you back to disbelief after you have believed, out of envy from themselves [even] after the truth has become clear to them. So pardon and overlook until Allah delivers His command. Indeed, Allah is over all things competent.} [Quran 2:109].
We the Muslims are proud of our religion that was revealed to us by Allah and are thankful that Allah has chosen us for his religion.
Allah Says (what means): {…This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion…} [Quran 5:3]
The non-Muslims’ celebrations such as the second millenium, Christmas Day, etc. are among the falsehoods that a Muslim should avoid. Allah Says (what means): {And [they are] those who do not testify to falsehood, and when they pass near ill speech, they pass by with dignity.} [Quran 25:72].
Many known ancestors made clear that the “falsehood” in the above verse means feasts of non-Muslims. Imam Ahmad in his book of Hadeeth book (Musnad) and Imam Tirmithi in his book of Sunan reported on the authority of Anas who said that the Prophet, sallallaahu ‘alayhi wa sallam, came to Madeenah while people there had two days in which they played. The Prophet, sallallaahu ‘alayhi wa sallam, asked, “What are these two days?” They said, “These are two days we used to play in, in our Jahiliyyah! The Prophet said, “Allah has replaced them with two better days. ‘Eed al-Adhha and Eed Al-Fitr”.
A person came to the Prophet, sallallaahu ‘alayhi wa sallam, and said: “I have taken a vow to sacrifice a camel at Bu’wanah. The Prophet, sallallaahu ‘alayhi wa sallam, asked: Did the place contain any idol worshipped in pre-Islamic times? They (the people) said: No. He asked: Was any pre-Islamic festival observed there? They replied: No. The Prophet, sallallaahu ‘alayhi wa sallam, said: Fulfil your vow, for a vow to do an act of disobedience to Allah must not be fulfilled, neither must one do something over which a human being has no control.”
No doubt, celebrating such feasts is actually imitating non-Muslims. The companion of the Prophet ‘Abdullah Bin ‘Amar bin al-'Aas said: 'Whoever celebrates the Persian New Year's Day or their carnival and imitates them till his death he will be resurrected with them on the Day of Judgment'.
The Prophet, sallallaahu ‘alayhi wa sallam, said as reported in a sound Hadeeth: "Whoever imitates a people he is one of them" [Ahmad and Abu Dawood].
Therefore, a Muslim is neither allowed to celebrate the Christmas Day nor is he allowed to congratulate them. Nor, does a Muslim celebrate any of their other feast days. A Muslim celebrates only the Islamic ‘Eed days.
Many times Allah has mentioned the details of the birth of Jesus in the Quran but He did not mention any celebration to be done on this occasion. For more benefit, please read the Fatwa 82006.
Allah knows best.