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.

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

Adha, the festival of sacrifice by offering prayers in mosques and open spaces before sacrificing animals. (AP video by Muhammad Farooq, Shonal Ganguly and Al Emrun Garjon)