This event is very significant in Islamic history, No one can ignore it, hide it, and or deny it. 
Over 110 Sahabi Narrators of this important tradition (Hadith Ghadir Khumm), according to Sunni Sources, cited this important event. This is the day, where, Holy Prophet (SAWA) appointed and declared that Al-Imam Ali (AS) is his Successor, after him.

This important and historic occasion took place on Thursday, 18th Thu-l Hijja, year 10 of Hijra (Thursday, March 21, 632 AD **), exactly 1434 Lunar years ago (1391 solar years ago), in a place called Ghadir Khumm.  Ghadir Khumm is a place that included a water well around which rain water also gathered and is approximately three miles from present-day "Juhfeh," District of Rabegh, between Mecca and Medina (Juhfeh is a halting place where pilgrims put on their Haj robes)…… 

**) The exact date is between March 12 and March 25, but, most likely was on March 21, 632 AD.

Holy Prophet was returning back from Mecca, after his last and farewell pilgrimage (Hijjatul- Wada') to House of Allah (Al-Ka'ba) and when he stopped at Ghadir Khumm together with over 70,000 pilgrims, Allah revealed to him to declare and appoint Al-Imam Ali (AS) as his Successor and the Leader (Imam) for all Muslims, after him. At Ghadir Khumm, five hours after sunrise, Angle Gabriel (AS) descended and after  salutations to the Holy Prophet, he recited the following verse from the Holy Qur'an:

"O Messenger! Deliver what has been revealed to you (with regard to Ali) from your lord (Nourisher); and if you do it not, then you have not delivered His complete message, and Allah shall protect you from the people; surely Allah shall not guide the unbelieving people." (Verse 67, Sureh 5: Al-Ma'edeh)

This urgent command of Allah was revealed to him after nearly 23 years of hard work, prophesizing  and only about 70 days before his death.  Upon this very important command of Allah, the Holy Prophet (SAWA) declared and appointed Al-Imam Ali, in the presence of over 70,000 of his followers, to be his Successor and the Leader, after him. He also asked the gathered people for their true allegiance to  Al-Imam Ali, the Commander of Faithful (Amir-ul-Mu'menin), as well as, he obtained their oath of allegiance to Al-Imam Ali(AS).  He told them:

"Allah Almighty and Glorious has appointed Ali to this position amongst you, after me."

This is the day, where Allah (Subhanehu wa-Ta'la) has perfected the  Islamic religion and has  completed His blessings. 

After, only about 70 days from this event, the Holy Prophet Muhammad (SAWA) passed away. Some of  his close followers after his departure betrayed him and disobeyed his and Allah's orders. Things changed and Al-Imam Ali (as) was prevented from assuming his righteous role as the Successor of Holy Prophet Muhammad (SAWA) and the  religious Leader of the Islam. This was an inflection point in Islamic history from truth and justice into corruption, which is continued until now.

In conclusion, the event of Al-Ghadir is the pivot of Islam, where the truth turns around it and rotates about it.  Al-Imam Ali is the  focal point of this event. The truth will turn around and around  continuously until the day of appearance of Al-Imam Al-Mehdi (AS) from his occultation, who is the true final Leader in this universal lineage .  The power of this event will destroy, burn, and perish falsehood. Because, falsehood is bound to perish.  And the truth and justice will prevail.  At that time, humanity will see the light of Allah and enjoy the freedom and justice under the rule of Al-Imam Al-Mehdi (AS)…

AS = Alayhis-Salam = Peace and Blessings Be Upon Him
SAWA = Sallalahu Alayhe Wa Alehe Wasallam = Peace and Blessings Be Upon Him and His Family