Every people have their own civilization, their culture and days of celebration. Allah and His Messenger (saws) have appointed the two Eids for celebration in Islam, and have made every Friday a day of Eid and celebration for the believers. For a true believer, every day that dawns is a Sign from our Merciful Lord, and every day that Allah gives us life, can be a day of celebration for us.

There is absolutely no harm if one celebrates or congratulates ones non-muslim friends and brethren in humanity on their personal accomplishments and happy occasions like their marriage, the birth of their baby, or their childrens graduation, or a promotion in their job, etc. But it would neither behove nor befit a believer who believes in Allah and the Last Day to greet or celebrate any of their religious occasions with them like Christmas, Easter, Diwali, Holi, etcfor that then would be to intentionally witness a falsehood.

Allah Says in the Holy Quran Chapter 25 Surah Furqaan verse 72:
72 (The true slave-servants of Allah are) Those who witness no falsehood (intentionally), and if they pass by futility they pass by it with honorable (avoidance)

In light of the guidance of the Quran and the Sunnah, the jurors in Islam have listed four categories which would make the participation of any celebration forbidden:
The celebration should not be a pagan religious festival.
The celebration should not involve any shirk.
The celebration should not involve or encourage fahisha or indecency or any of the things absolutely forbidden in Islam (alcohol, gambling, etc.)
The celebration should not indulge in excess spending.

The pagan beliefs and religions and thus their religious festivals are based on absolute falsehoods, and it does not behove one who sincerely believes in Allah and the Last Day as Truth to indulge in celebrations of falsehood. To greet or celebrate pagans on their pagan and religious days of festivals would be to justify their false beliefs.

Allah Says in the Holy Quran Chapter 5 Surah Maidah verse 2 (part):
.Help ye one another in righteousness and piety, but help ye not one another in sin and rancor: fear Allah: for Allah is strict in punishment.

Sunan of Abu-Dawood Hadith 4020 Narrated by Abdullah ibn Umar
The Prophet (saws) said: He who copies any people is one of them.

Al-Tirmidhi Hadith 4649 Narrated by Abdullah ibn Amr ibn al-'As
Allah's Messenger (saws) said, "He does not belong to us who imitates other people.

It would be absolutely forbidden for the believers who sincerely believe in Allah and the Last Day to greet or participate in any way whatsoever in the religious festivals of the pagans.

One cannot say with absolute conviction that wishing one another Happy New Year on the 1st of January is based on a pagan religion.but without an iota of a doubt it would be absolutely forbidden for the believers to wish ones Chrisitan brethren Merry Christmas..for they claim and believe (and I seek refuge in Allah from even saying this!) that the Noble Messenger of Allah (saws) Isa ibn Maryam (a.s.) is the son of God and claim that he was born on this day!

If one trusts, obeys, and follows the guidance and commands of Allah and His Messenger (saws), one can be assured of never ever being misled; but if one believes, obeys and follows any other guidance, other than that of Allah and His Messenger (saws), one can be assured of being led astray.

Whatever written of Truth and benefit is only due to Allahs Assistance and Guidance, and whatever of error is of me alone. Allah Alone Knows Best and He is the Only Source of Strength.