On 18th Dhu-l-hijja of the year 10 A.H. (10 March 632 C.E.), after completing the last pilgrimage, the Holy Prophet (PBUH) along with around 100,000 Muslims reached a place called 'Ghadir-e-khumm' near Mecca. The following verse was revealed: [Quran 5:67] O Messenger! Deliver what has been revealed to you from your Lord; and if you do it not, then you have not delivered His message, and Allah will protect you from the people; surely Allah will not guide the unbelieving people.

On receiving the above verse, the Holy Prophet stopped at that very place and delivered a speech an extract of which is :-[....It seems the time has approached when I shall be called away (by Allah) and I shall answer that call. I am leaving for you two precious things and if you adhere to them both, you will never go astray after me. They are the Book of Allah and my Progeny that is my Ahlul Bayt. The two shall never separate from each other until they come to me by the Pool (of Paradise).'' Then the Holy Prophet (PBUH) in an attempt to remind Muslims of his own authority over them said: ''Do I not have more right over the believers than what they have over themselves?'' Muslims answered unanimously ''Yes, O' Messenger of God.'' This served as a stepping stone for the announcement of his heir and vicegerent. The Holy Prophet (PBUH) held out the hand of Ali (AS) and said: ''For whomever I am his Leader (Mawla), Ali is his Leader (Mawla).'' The Holy Prophet then continued to say: ''O' God, love those who love him, and be hostile to those who are hostile to him.'']

This day is also known as 'Ahad e-Mahood' in the heavens and as 'Meethaq-e-Maakoodh' on the earth

Eid-e Ghadir, also known as Eid al-Ghadir Khumm, is an anniversary of special significance to all Shia Muslims. It commemorates the occasion at Ghadir Khumm when, by divine command, Prophet Muhammad (peace be upon him and his family) designated his cousin Hazrat Ali — husband of his daughter and only surviving child, Hazrat Bibi Fatima — as the first in the continuing line of hereditary Imams. The occasion of Ghadir Khumm is also associated with the well-attested Prophetic tradition that he was leaving behind two weighty things: the Holy Qur’an and his progeny, which the Shi’a consider to be the Prophet’s testamentary declaration.

Numerous reliable hadith sources — both Shia and Sunni — record the event at Ghadir Khumm. They agree that the Holy Prophet, on his return journey from the final pilgrimage, stopped at an oasis between Mecca and Medina known as Ghadir Khumm, and addressed the large gathering of Muslims who were accompanying him. The Prophet asked them if he had a greater claim upon them than they had upon themselves, to which the Muslims responded affirmatively. The Prophet then went on to say: ''He whose Mawla I am, Ali is his Mawla… Oh Allah, help whoever helps him, oppose whoever opposes him, support whoever supports him, forsake whoever forsakes him, and may the truth follow him wheresoever he turns.''

The Shia Ismā'īlī tradition bears witness to the continuity of the authority vested at Ghadir Khumm. The hereditary Imamah has continued over 1,400 years, from Hazrat Ali to the present Imam-of-the-Time, Mawlana Shah Karim al-Hussaini Aga Khan, who is the 49th hereditary Imam and direct descendant of Prophet Muhammad through Hazrat Ali and Hazrat Bibi Fatima.

In commemorating Eid-e Ghadir, the Jamat celebrates the seminal event of Ghadir Khumm, also reaffirming their allegiance to the Imam-of-the-Time as the direct lineal successor and inheritor of the authority of Hazrat Ali. 

Source: Islamiclife