Praise be to Allaah.
It is not permissible to celebrate innovated festivals such as the Christmas of the Christians, or Nowrooz (Persian New Year) or Mahrajaan (Persian festival), or festivals that have been innovated by Muslims such as the Prophet's birthday in Rabee' al-Awwal or the Israa' in Rajab and so on. It is not permissible to eat from that food which the Christians or Mushrikeen prepare on the occasion of their festivals. It is also not permissible to accept their invitations to join them in their celebrations of those festivals, because this encourages them and is tantamount to approving of their bid'ah, which gives the wrong idea to ignorant people and makes them think that there is nothing wrong with that. And Allaah knows best.
Reproduced from Islam QA