Eid al-Ghadir - The most important of all Eids in Islam
Towards the end of his life, Rasulollah (the Messenger of Allah) - Muhammad, peace be upon him (PBUH) was ordered by Allah Almighty to publicly and officially appoint Imam Ali as his successor and to seek the Muslims allegiance for Ali (PBUH).
The Ayah or verse that instructs Rasulollah to do so is
O Apostle! Proclaim that which has been sent to you from your Lord. If you do not, you would not have fulfilled and proclaimed His Mission. And Allah will defend you from the people (who mean mischief). The Holy Qur'an 5:67.
This noble Ayah came down on Rasulollah during his return from the hajj, on the 18th Day of the month of Dhil-Hijjah, year 10 AH, i.e. only 70 days before his martyrdom. Many tens of thousands of Muslims have been on Hajj with Rasulollah that year. When the ayah was revealed upon Rasulollah (peace be upon him) he ordered those Muslims who were ahead of him to come back and waited for those were behind him to arrive at the area he was standing. Rasulollah and the tens of thousands of Muslims (some reports estimate them to be more than 120,000 ) assembled in an area called Ghadir Khum. It was at that area when Rasulollah conveyed the Message to the Muslims and said in his speech:
"Whomever I am his master, Ali is his master too. He is my Caliph (successor) upon you after me.
O Lord support whoever supports Ali and oppose whoever opposes him."
Rasulollah (peace be upon him) then ordered all the Muslims to make Bay'ah or give allegiance to Imam Ali and address Ali as Ameer-ul-Mu'meneen which means Commander of the Faithful.
The title Ameer-ul-Mu'meneen was exclusively granted to Ali by Rasulollah on instructions from the Almighty.
All male Muslims made Bayah with Ali as Ameer-ul-Mu'meneen by shaking hands with him and congratulating him on this great honour, including Abu Bakr and Omar. Abu Bakr and Omar are famous for their well-known address to Imam Ali on this occasion when each of them said to Ameer-ul-Mumeneen, Ali (PBUH):
BAKHIN, BAKHIN, (Bravo, Well done) for you have become my Commander, and the Commander of every Muslim and Muslimah.
The female Muslims were also ordered by Rasulollah (peace be upon him) to give this Bay'ah but instead of shaking hands with Ameer-ul-Mu'meneen, which would have been haram, each of the Muslim ladies immersed her hand in a vessel filled with water, into which Imam Ali had earlier dipped his hand in. This command of Allah Almighty is an obligation, which every Muslim remains obliged to honour as long as s/he lives, and on the day of judgement will be questioned about his/her adherence to it, for Allah Almighty says, "But stop them, for they must be asked. The Holy Quran 37:24.
After the death of Rasulollah (peace be upon him), some Muslims did not observe or adhere to the Bayah that they had given to Ameer-ul-Mu'meneen and some did. Those who did adhere to Allah's command and obeyed the order of Rasulollah became known as the Shia.