Shi'is worldwide mark Eid al-Ghadir with celebration and special prayers, commemorating the life and role of Imam 'Ali. They reaffirm their identity as Shi'is, Muslims who hold line of the Prophet's family descending through Imam 'Ali and his wife Fatima, Muhammad's daughter, (Ahlul Bayt) in special regard. The majority (of Shi'is) Twelver Shi'is, Isma'ilis, and Zaydis all believe in the spiritual and communal/political authority of the first five, and in the case of the Isma'ilis six, Imams, male members of the Ahlul Bayt who they view as historical and, in some cases, current-if-occulted heads of the Muslim community. After the sixth, they believe in different lines of Imams.
In some mixed areas, such as Iraq and Yemen, where both Shi'is and Sunnis live, Eid al-Ghadir is a period of communal tension.
(1) "Event of Ghadir Khumm in the Qur'an, Hadith, History"
(2) "Orientalists and Ghadir Khumm"
(3) Ghadir Khumm in Sunni and Shi'i Texts, with References
Read two Isma'ili views of the event:
(1) "The Imamate in Isma'ilism"
(2) Ghadir Khumm
In the interest of balance, read an article about Ghadir Khumm from a critical Sunni perspective, generally speaking. It includes refutations of the Twelver Shi'i interpretation as well as sectarian inuendo common in such polemical exchanges between Shi'is and Sunnis, from both groups. It is somewhat difficult to find critical Sunni views of this event, as the bulk of material that initially turns up in most searches is Shi'i hagiographical material.
(1) "Hadith of Ghadir Khumm: A Sunni Perspective"