Eid al-Ghadeer is an Islamic holiday observed by Shia Muslims on the 18th day of the month of Dhu al-Hijjah.
It celebrates the appointment of Ali ibn Abi Talib as the successor of the Islamic prophet Muhammad.
Before his death, Muhammad made the Farewell Pilgrimage, his last pilgrimage from Medina to Mecca.
After giving the Farewell Sermon atop Mount Arafat, Muhammad descended and started back towards Medina.
On the way back, he made a stop at the pond of Khumm and appointed Ali ibn Abi Talib, his cousin and son-in-law, the master of believers.
The interpretation of Ali's role is a principal disagreement in Shia-Sunni relations.
Shias celebrate this day because they consider Ali the first Imam after Muhammad.
