The event of Ghadir Khumm (Arabic and Persian: واقعه غدیر خم) is an event that took place In 18th of al-Hijah 10th AH . While returning from the Hajj pilgrimage, the Islamic prophet Muhammad gathered all the Muslims who were with him and gave a long sermon. This sermon included Muhammad's declaration that "to whomsoever I am Mawla, Ali is also their Mawla." After the sermon, Muhammad instructed everyone to pledge allegiance to Ali.
Shia Muslims believe this event to be the official appointment of Ali as Muhammad's successor. The day's anniversary in the Islamic calendar (18 Dhu al-Hijjah) is celebrated by Muslims (primarily Shias) as Eid al-Ghadeer.