The Prophet (s.a.w.a.) began his journey from Medina toward Mecca for Hajj on Dhu l-Qa'da 24 or 25 of 10/632 AC accompanied by thousands of people. Since that hajj was the last hajj of the Prophet, it was later called "Hajjat al-Wida'". When hajj rituals finished and the Prophet left Mecca toward Medina, they arrived at Ghadir Khumm, on Dhu l-Hijja 18 and in this place; Gabriel revealed the verse of Tabligh (5:67) to the Prophet and delivered him the order of God for appointment of Ali (a.s.) as Wali and Wasi (successor) after the Prophet (s.a.w.a.) and announcing it to people.
According to hadiths, the Prophet (PBUH) gathered people in Ghadir Khumm, raised Ali's (a.s.) hand so that everyone sees and asked, "O people! Do I not have wilaya (guardianship) over you more than you have over yourself?" People answered, "Yes, O Prophet (s.a.w.a.)!" Then, the Prophet (s.a.w.a.) said:
"God is my wali (guardian) and I am wali of the believers; and I have wilaya over them more than they have over themselves. So anyone I am his wali, Ali (s.a.w.a.) is his wali."
Then, the Prophet repeated it three times and said, "O God! Befriend and be the wali of anyone who loves Ali and considers him as his wali; be the enemy of anyone who is his enemy; and assist anyone who assists him and abandon anyone who abandons him."
He then told people, "Those who are present, convey this message to those who are absent."
In Hadiths
In Sunni references, there is a hadith saying that, "anyone who fasts Dhu l-Hijja 18, God regards the rewards of 6 months fasting for him" and that "this day is Eid Ghadir Khumm."
The Prophet (PBUH) said,
"the day of Ghadir Khumm is the best Eid of my Ummah (people) and that is the day God the Almighty ordered to appoint my brother Ali b. Abi Talib (PBUH) as the standard-bearer of my Ummah; so that, people will be guided by him after I pass away; and that is the day God perfected the religion and completed blessings upon my Ummah and chose Islam as their religion."
Also, Imam al-Sadiq (a.s.) said,
"The day of Ghadir Khumm is the grand Eid of God. God has not sent any prophet unless he has celebrated this Eid, and has glorified it. And its name in the heavens is the day of covenant and on the earth is the day of solemn covenant and public presence."
In another hadith from Imam al-Sadiq (PBUH), he regarded Eid al-Ghadir the greatest and most glorious Eid of Muslims, in every hour of which, they need to thank God; and that people fast the day to show their gratitude and that fasting the day equals 60 years of worship."
Imam al-Ridha (a.s.) said,
"The Day of Ghadir is more popular among the dwellers of heavens than among the people of the earth… If people knew the value of this day, undoubtedly angels would shake hands with them ten times a day."
History of the Celebration
Muslims, especially Shi'a consider the day of Ghadir as one of the greatest eids since the early Islam and this day was famous among them as Eid al-Ghadir.
In his book, Mas'udi (d. 346/957-958) wrote that children and followers of Imam Ali (a.s.) venerated this day. In a hadith, al-Kulayni (d. 328/940) has narrated Shi'a's celebration of this day.Therefore, it is clear that celebration of Eid al-Ghadir has been common in the 3rd/9th and 4th/10th centuries.
Earlier, Fayyad b. Muhammad b. 'Umar al-Tusi narrated a tradition which suggested that Imam al-Ridha (PBUH) celebrated the day of Ghadir. Since Imam al-Ridha (a.s.) was living at the end of the second/ early 9th century , it becomes clear that the celebrating of the day of Ghadir goes back early centuries of Islam.
Such a celebration has been common among Muslims in the following years and centuries, so that the ceremony of allegiance with al-Musta'la b. Mustansir (one of the rulers of Egypt) was held on the day of Eid al-Ghadir 487/1094. Fatimid caliphs made the Eid al-Ghadir an official Eid in Egypt; and in Iran, it has been among official holidays since 907/1502, when Isma'il I, the Safavid king came to throne.
In recent centuries, celebrating this day has been considered among the symbols of Shi'a. In Najaf, a great celebration is held every year in the courtyard of Imam Ali's (a.s.) holy shrine and noblemen and scholars of Shi'a and ambassadors of Islamic countries attend there and great speeches and odes are recited. In Yemen, Zaydi Shi'a celebrate the day of Ghadir and illuminate and decorate their streets.
The eve of Eid al-Ghadir is also among the important nights among Muslims.