From the Mawlid-al-Nabi Wikipedia page:
Mawlid is the observance of the birthday of the Islamic prophet Muhammad which is celebrated often on the 12th day of Rabi' al-awwal, the third month in the Islamic calendar. The 12th Day of Rabi' al-awwall is the most popular date from a list of many dates that are reported as the birth date.
The origin of Mawlid observance reportedly dates back to the period of the early four Rashidun Caliphs of Islam. The Ottomans declared it an official holiday in 1588. The term Mawlid is also used in some parts of the world, such as Egypt, as a generic term for the birthday celebrations of other historical religious figures such as Sufi saints.
Most denominations of Islam approve of the commemoration of Muhammad's birthday; however, some denominations including Wahhabism/Salafism, Deobandism and the Ahmadiyya disapprove its commemoration, considering it an unnecessary religious innovation (bid'ah or bidat). Mawlid is recognized as a national holiday in most of the Muslim-majority countries of the world except Saudi Arabia and Qatar which are officially Wahhabi/Salafi.