Answer:
All devout and able Muslims attempt a hajj (pilgrimage) to Mecca at least once in their lifetime. Because it is sacred, only Muslims are allowed to enter the city. ... As a result, Mecca can accommodate the continually increasing number of pilgrims, or hajjis.
As the birthplace of Muhammad and the site of Muhammad's first revelation of the Quran (specifically, a cave 3 km (2 mi) from Mecca), Mecca is regarded as the holiest city in the religion of Islam and a pilgrimage to it known as the Hajj is obligatory for all able Muslims.
