An Egyptian Sufi Muslim practices ritualized Zikr (invocation) to celebrate Mawlid an-Nabi, or the birth of the Prophet Muhammad, in Cairo, Jan. 3, 2015. Reuters/Amr Abdallah Dalsh

Muslims generally celebrate the birth of the Prophet Muhammad on the 12th day of the third month of the Islamic calendar, called Rabi’ al-Awwal. Because the lunar calendar advances 11 days each year, the prophet’s birthday changes year to year, and falls on Wednesday this year. It begins Tuesday at sunset, and Shiites generally celebrate a few days later.

Not all Muslims celebrate the day, however. Mawlid al-Nabi (“the Prophet’s Birthday"), as it is called in many countries, is often associated with Sufi strains of Islam. Some countries, like Saudi Arabia, do not publicly recognize the holiday, viewing it as an unnecessary religious innovation.

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People buy traditional sweets for children to celebrate the birthday of the Prophet Muhammad, also known as Mawlid an-Nabi, in a makeshift tent in Cairo, Dec. 30, 2014. REUTERS/Mohamed Abd El Ghany

In much of the Muslim world, the date is marked with songs and poems in honor of the Prophet Muhammad. A famous 13th-century poem, "Qasida Burda," is often read in praise of the prophet and the mercy he is believed to have brought. Cities are often decorated, and many people hand out sweets and candy to children. Traditions vary by country.

Muhammad is believed to have been a descendant of Ishmael. Both Isaac and Ishmael were children of Abraham, according to Judaism, Christianity and Islam. Whereas Jews are believed to have descended from Isaac, and the Bible generally follows his lineage, Arabs are believed to have descended from Ishmael.

Muhammad is believed to have been born in Mecca in the year 570 in the tribe of Quraysh. It was customary among wealthy Quraysh to entrust newborns to country women, whose lifestyles were deemed more pure and their Arabic of a higher standard. Muhammad was reportedly given to Halima Sa’dia, who, according to Islamic tradition, witnessed great blessings by taking in the child.

Malaysian Muslim students recite prayers ahead of the Mawlid celebrations in Putrajaya, Feb. 11, 2011. REUTERS/Bazuki Muhammad

His mother Aminah died when Muhammad was 6, and his father was already dead by then. The orphan was protected by his uncle Abu Talib and his grandfather Abdul-Muttalib. That he was an orphan plays strongly into his biography, and is mentioned in the Quran. Abu Talib, who held sway among the Quraysh, protected Muhammad after his tribe turned against him when he declared himself a prophet decades later. Abu Talib himself never converted to Islam.

At a young age, Muhammad started learning about business from his uncle. He was spoken about highly, and became a successful and trusted trader. His tribesmen referred to him as “the trustworthy one.” According to Islamic tradition, it was while 12-year-old Muhammad was on a caravan trip with his uncle that a righteous Christian monk, Bahira, informed Abu Talib that his nephew would become a great prophet. “This is the master of all humans, Allah will send him with a message which would be a mercy to all humans,” Bahira said.

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People carry an offering called gunungan during a ritual to commemorate the Prophet Muhammad's birthday in Yogyakarta on the Indonesian island of Java, March 9, 2009. REUTERS/Dwi Oblo

It was not until Muhammad was 40 years old that he was commanded by the angel Gabriel, who carried God's message, to declare God's oneness and to shun the idolatry of his tribesmen. The earliest revelation said: “Read in the name of thy lord and cherisher who created: Created man out of a clot of congealed blood. Read! And thy lord is most bountiful, He who taught the use of pen; Taught man that which he knew not.”