What is the ruling on celebrating the Prophet's Birthday (Mawlid al-Nabawi)?

Celebrating the Prophet's Birthday (Mawlid) is a reprehensible innovation (bid'ah munkarah) that was not practiced by the Prophet (peace be upon him), nor his Companions, nor the Tabi'un (followers of the Companions). Had it been good, they would have done it. It was originated by the Fatimids in the sixth century Hijri, and it is among the newly invented matters forbidden in Islam, for every innovation is a bid'ah, and every bid'ah is misguidance.