Ghadir Khom

Every eighteenth of Dhul-Hijjah is marked and celebrated world-wide by the Shia-Muslims as the Ghadir Khum festival. The day is a remarkable day in the history of Islam as the Apostle of God (Peace be upon him and his pure household) was commanded on this day in the tenth year after Hijrah to stop at the pond of Khumm with the sole aim of nominating Ali son of Abu-Talib (peace be upon him) as his vicegerent and the immediate successor after him. With this divine appointment, the religion of Islam was perfected and approved of by Allah and His favours were completed.
 

یا اَیٌّها الرَّسُولَ بَلِّغْ ما اُنْزِلَ اِلیکَ مِنْ رَبِّکَِ؛ المائده:67

قال رسول الله صلی الله علیه و آله:مَنْ كُنْتُ مَوْلاهُ، فَهذا عَلِىٌّ مَوْلاهُ

Muslims in medina did not support Imam Ali (A) against usurpers as if they forgot the word of the Prophet (P) about Ali (A) and his sermon in Ghadir. Several protests have been registered by him indicating that had Imam had real and sufficient supporter he would have waged a war against usurpers.

Imam Ali (A) was appointed as the Prophet's successor in Ghadir Khumm, but some of the companions opposed him due to some reasons that are going to be mentioned.