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.

On Friday Muslims will begin observing Eid al-Adha, a time for prayer and charity. The religious festival occurs over three to four days and is one of the major Islamic celebrations observed by Muslims every year.