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: