There has been a debate in my country about the permission to give congratulatory message to our non-Muslim friends during their time of celebration (Christmas, Passover, etc) because some preachers here consider congratulating them as similar to approving their faith and thus rejecting the truthfulness of Islam. Is there a clear fatwa on this issue? Because, I have several non-Muslim friends who are going to celebrate their religious special days and I have not congratulated them, as they have always congratulated me during any Islamic celebrations.
It is different that a Muslim congratulating the celebration days of non-Muslims himself and congratulating the non-Muslim neighbors or friends on their celebration days.
It is not permissible for a Muslim to celebrate the religious days of non-Muslims, such as Christmas, Easter, passover etc. On the other hand, it is not wrong for a Muslim to wish those days to bring goodness to his non-Muslim acquaintance. Congratulating them on these special days does not mean internalizing those days. This is considered as for setting up good relations with non-Muslims.
For further information, see:
Relations with Non-Muslims: http://www.islamandquran.org/?p=1201