Eid al-Ghadir is a major religious holiday for Shia Muslims, commemorating the appointment of Ali ibn Abi Talib as the successor of the Prophet Muhammad. It's held on the 18th of Dhu al-Hijjah, and is considered by many Shia Muslims to be one of the most important religious celebrations. The name "Ghadir" refers to the pond at Ghadir Khumm, where Muhammad is believed to have made the announcement, according to Shia Islamic beliefs.