The Event of Ghadir (Arabic: واقعة الغدير), according to Shia beliefs, is among the most important events in the history of Islam, in which on his return from Hajjat al-Wida', the Holy Prophet (S) introduced Imam Ali (A.S) as his vicegerent and people's guardian after himself at a place called Ghadir Khum, where everyone including greatest Companions pledged their allegiance with Imam Ali (A.S).
This introduction was according to divine command in Tabligh Verse (the 67th verse of Surat al-Ma'ida) which was revealed a little earlier than Dhu l-Hijja 18, 10/March 19, 632 commanding the Holy Prophet (S) to communicate to people whatever has been revealed to him and if he does not, it is as if he has not fulfilled his mission. Subsequently, the Ikmal Verse (the third verse of Sura al-Ma'ida) was revealed saying: "Today I have perfected your religion for you, and I have completed My blessing upon you, and I have approved Islam as your religion." (Q 5:3).
The Infallibles (A.S) have referred to Ghadir Sermon and many poets -from the time of Imam Ali (a) onward- have composed poems about this event. Among the most prominent works about the Event of Ghadir and Ghadir Sermon is Al-Ghadir fil-kitab wal-sunna wal-adab by al-'Allama Amini (الغدير في الکتاب و السنة و الأدب). The Holy Prophet (S) and other infallibles (A.S) have called this day Eid (Holy Day) and Muslims, especially the Shia, celebrate it.
Read more about the Story of Ghadir HERE.