The Hajj leading up to Eid al-Adha is one of the five pillars of Islam. The pilgrimage is one of the largest religious gatherings in the world. Mecca, considered the holiest city in Islam, is the site of Muhammad’s first revelation of the Quran.

Large numbers of Muslims converge once a year on Saudi Arabia to unite in a series of religious rituals and acts of worship as they perform the Hajj, the pilgrimage that is one of the pillars of Islam.

Adha, a period of "communal enjoyment" for Muslims worldwide, has officially arrived. Here's what we've learned about the Islamic holiday.