Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.

 

Saluting others with messages on celebration wishes of religious festivals tacitly includes an act of encouraging them to do those festivals which are mostly dedicated for their false deities.

 

In Islam, a Muslim does not have a double standard. He has a firm belief that what those people do is polytheism which is greatly disliked by Allah. Therefore, celebrating those minorities of those religious occasions and festivals is not permissible.

 

Congratulating them for their social occasions or for their personal successes is permissible. This is a part of being just and fair to non-Muslims.

 

Almighty Allah knows best.