"The Eid of the Khilafat and Wilayat"
It has been narrated from Ziyad ibn Muhammad that he said, "I went to see Abi 'Abdillah [Ja'far ibn Muhammad as-Sadiq] (peace be upon him) and said to him, "Do the Muslims have an 'Eid other than the day of Jumu'ah and al-Fitr and al-Adhha?"
The Imam (peace be upon him) replied to me,
" Yes, the day which the Messenger of Allah (blessings of Allah be upon him and his family) appointed Amirul Mo'minin (peace be upon him) [as the leader of the Ummah after him]."
(Reference: Misbahul Mutahajjid, Page 736)
The Best 'Eid of the Ummah
The Messenger of Allah (blessings of Allah be upon him and his family) has said:
" The day of Ghadir Khumm is the best 'Eid of my nation. It is the day on which Allah, the High, ordered me to mention the appointment of my brother, 'Ali ibn Abi Talib as the flag and standard of my nation. People shall be guided by him after me and this is the day in which Allah completed the religion and perfected the bounties upon my nation and on which He was pleased with Islam as their religion."
(Reference: Al-Amali of as-Saduq, Page 125, Hadith 8)
The Great 'Eid of Allah
It has been narrated from as-Sadiq [Imam Ja'far ibn Muhammad] (peace be upon him) that he said,
" This is the greatest 'Eid of Allah, and every Prophet appointed by Allah celebrated this day as an 'Eid, and knew the sanctity of this day. The name of this day in the heavens is the Day of the Allegiance and Well-Known Pact, and on the Earth this Day is known as the Day of the Promise which is taken and the Day of the Gathering of those who witnessed (the event)."
(Reference: Wasa'il ash-Shi'a, Volume 5, Page 224, Hadith 1)