Why Eid Al-Ghadeer is called the greatest Eid of Allah?

Eid Al-Ghadeer is called "the greatest Eid of Allah" for its great importance, as in the Eighteenth of the holy month of Dhu Al-Hijjah in the tenth year of the Prophetic migration, there was the announcement of very important event, as the Almighty Allah has stressed on His holy Messenger (Allah's prayers be upon him and upon his holy Household) to announce that which has been revealed to him to the Muslims in this place, and did not allow him to postpone it until Medina, as the Almighty revealed: {O Messenger, announce that which has been revealed to you from your Lord, and if you do not, then you have not conveyed His message. And Allah will protect you from the people. Indeed, Allah does not guide the disbelieving people.} (5:67)

The divine order came so the Messenger of Allah (prayers and Allah's peace be upon him and his holy Household) nominate Ali (peace be upon him) Imam for the people, the successor and the guardian after him, and to inform the public about the revelation of his "Wilayah" to impose his obedience on every one.

And it is called the greatest Eid of Allah because on this day the Almighty God has perfected the religion and completed the favor: {This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion.} (5:3)

It is called the greatest Eid of Allah because on this day the faith has been completed, referring to the following verse: {The desert Arabs say, "We have believed." Say, "You have not believed; but say [instead], 'We became Muslim,' for faith has not yet entered your hearts. And if you obey Allah and His Messenger, He will not deprive you from your deeds of anything. Indeed, Allah is Forgiving and Merciful."} (49:14)

Thus, Al-Walayah is the perfection of the faith as the Messenger of Allah (Allah's prayers be upon him and upon his holy Household) has said about the Commander of the Faithful (peace be upon him) in the Battle of the Trench [Khandaq]: "The whole faith [Imaan] has confronted the whole polytheism [Shirk]."

Eid Al-Ghadeer is called the greatest Eid of Allah because all other Eids are celebrated by the hypocrites and the Muslims apart from Eid Al-Ghadeer, which is celebrated only by believers in affirmation of the word of the Messenger of Allah (Allah's prayers be upon him and upon his holy Household): "O Ali, only a believer will love you and only a hypocrite will hate you."