Factbox-What is the Muslim hajj pilgrimage?

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

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

* A quota system for visitors to Mecca was introduced in 1987, agreed by member countrie