Eid al-Ghadeer is the anniversary commemorating the holy Prophet Muhammad's last sermon at Ghadir Khumm, which occurred on 18th of Dhu al-Hijjah of 10 AH in the Islamic calendar. It is an event celebrated by Shias, and is the basis of their confirmation of Ali ibn Abi Talib's succession to Muhammad. Sunni Muslims do not deny that a last sermon took place, but do not celebrate the day, as in they believe that, in Islam, the only days that are considered celebratory are Eid al-Fitr and Eid ul-Adha.

(... from Wikipedia on 2009-01-01 08:15:45 )