Factbox-What is the Muslim Haj pilgrimage?

DUBAI (Reuters) - Here is a look at the haj, one of the largest mass gatherings in the world, which starts in Saudi Arabia on Friday.

* The haj is an annual pilgrimage that millions of Muslims make to Mecca with the intention of performing religious rites as taught by the Prophet Mohammad to his followers 14 centuries ago.

* The fifth pillar of Islam, the haj is mandatory once in a lifetime for every able-bodied Muslim who can afford it and is the most significant manifestation of Islamic faith and unity.

* The haj begins in the 12th month of the Islamic year, which is lunar, not solar, meaning the haj and the fasting month of Ramadan can fall at different times of the solar calendar each year.

* A quota system for visitors to Mecca was introduced in 1987, agreed by member countries of the Organisation of Islamic Cooperation, to limit the number allowed by each country to 0.1% of its population.

* Stampedes, tent fires and other accidents have caused hundreds of deaths over the past 30 years, forcing the Saudi government to build new infrastructure. In 2011, Saudi Arabia began the biggest expansion yet of the Grand Mosque of Mecca to increase its capacity to 2 million. A new railway will link the holy sites around Mecca.

* Eid al-Adha, one of Islam's two main festivals, marks the climax of the annual haj pilgrimage, when Muslims slaughter animals - to commemorate the willingness of Ibrahim, or Abraham, to sacrifice his son on God’s command - often distributing meat to the poor.

(Reporting by Tala Ramadan and Michael Georgy; Editing by Bernadette Baum)