Ali bin Abi Talib (a.s.) during the Caliphate of Uthman enumerated his records in serving Islam and all of people were siting near him testified to the correctness of his words About the day of Ghadir Khum, Hazrat said: 
 In Ghadir Khum, the Prophet of God (pbuh) introduced me as follows: O people, do you know that God is my guardian and I am the guardian of you believers? And I am more worthy than you about your lives? They all replied: It is so, O Prophet of God.

Holy prophet on his return from the Hajj al-Weda' fulfilled his last mission. and based on the execution of God's orders and the introduction of his successor, Ali bin Abi Talib (AS), the verse of the completion of the religion was revealed, and Ghadir Day was intriduced as the celebration of guardianship.
 

-Pledge of Alliance in Ghadir shows that people must be obedient to the convenant of Allah.

Ghadir is in the Sunni books just they don't focus on it

Ghadir Eid is one of the most important Eids of Muslims in which the "Balagh" (delivering) verse was descended to the Holy Prophet and Islam religion was perfected. On this day, the Holy Prophet Mohammad ^(P.B.U.H.) introduced Imam Ali ^(P.B.U.H.) as his successor. Based on the narrations from the infallible ones ^(P.B.U.them), there are some recommended deeds and prayers for the Day of Ghadir.

The best Shiite Eid took place on the 18th day of Dhul-Hajeh and was named as Eid Ghadir Khum so that mankind may not have any argument against Allah and they would be saved from going astray

Ghadir

The main issue why I wrote this article is to explain and to tell Muslims eid Ghadir is not new but exists in Sunni books, not only Shia

Ghadir :And this incident took place in a situation where at that time in the history of Islam, it was unique in terms of the large number of people present in that scene, and the history of Islam had never seen anything like it until that day. 

One of the controversial discussions between Shia and Sunni commentators is the discussion about the cause of the revelation (Sha’n-e-Nozoul) of the verse 67 of chapter Ma’idah. Shias unanimously believe that this verse has been descended on the Day of Ghadir Khum and in Imam Ali’s honor, and the purpose of it is to express the Imamate and succession of Imam Ali (P.B.U.H.). By studying the sources of Sunnis, we realize that in various books of Sunnis, many narrations are found saying outspokenly: This verse has been descended about Imam Ali (P.B.U.H.).

-This article is about the questions we learn from Eid Ghadir and its values to the muslim Community.

this article continues to explain the things from Eid Ghadir, because of its importance in the religion of Islam, and since its the core of the message of the Holy Prophet.