There are only two Muslim festivals set down in Islamic law: Eid-ul-Fitr and Eid-ul-Adha (Eid or Id is a word meaning festival). But there are also several other special days which Muslims celebrate.
Some Muslims disapprove of celebrating the birthday of the Prophet (pbuh), on the grounds that it is an innovation, and innovations in religious matters are forbidden.
Some Muslims say that if changes were made in religious matters it would imply that Islam was not complete when it was revealed to the Prophet Muhammad (pbuh), or that the Prophet (pbuh) did not tell Muslims everything that was revealed to him. This would be seen as highly sacrilegious by many Muslims.