Celebrating the occasion of the Mawlid is a forbidden Bid`ah (innovation in religion). There is no evidence from the Qur'an or the Sunnah to support this act. It was not practiced by any of the four Rightly-Guided Caliphs. Moreover, the Prophet (peace be upon him) said:
Anyone who introduces anything into this matter of ours (Islam) that is not part of it will have it rejected.
He (peace be upon him) also said:
Adhere to my Sunnah (way) and the Sunnah of the righteous, Rightly-guided Caliphs. Hold fast to it and bite onto it with your molars (i.e. Cling firmly to it). Beware of newly-invented matters (in religion); for every newly-invented matter is a Bid‘ah, and every Bid‘ah is a Dalalah (deviation from what is right).
May Allah grant us success. May peace and blessings be upon our Prophet, his family and Companions.