There is nothing wrong with having celebrations on the occasions of marriage or other worldly occasions provided that they are free of reprehensible matters such as free mixing and music. Among the impermissible celebrations in Islam are those in which there is any imitation of the disbelievers such as birthday celebrations and Mother’s Day.

Celebrating Valentine’s Day is not permissible because it is an innovated festival and it promotes love and infatuation. For more, please see the detailed answer.

It is not permissible for the Muslims to exchange greetings on the occasion of the Gregorian New Year, and it is not permissible for them to celebrate it

It is not permissible for a Muslim to take part in the festivals and innovated celebrations of the non-Muslims, such as Easter and Christmas.