The knowledge of the Imam (as) in relation to hidden affairs was not intrinsic but something that Allah had bestowed on him. Such knowledge was placed at his disposal through different methods, the most prominent of which was inheritance from the Holy Prophet (pbuh) and his ancestors.

In a tradition (hadith) quoted from Imam Hadi (as), this point has been emphasized: Allah has not revealed His inner self to anyone but the Messenger (pbuh) in whom He is well pleased. So whatever knowledge the Messenger (pbuh) had, was what his successors knew as well. If the method was other than this the land of the Lord would be devoid of a knowledgeable messenger.

Quran 5:67
“O Messenger! Make known that which hath been revealed unto thee from thy Lord, for if thou do it not, thou wilt not have conveyed His message. Allah will protect thee from mankind. Lo! Allah guideth not the disbelieving folk."
The Gathering of all the pilgrims
The Messenger of Allah, The Holy Prophet Muhammad (pbuh) suddenly stopped and froze on hearing the above 'Ayah' from The Archangel Jibraeel (as).
The holy Prophet looked at his cousin Ali (as) intently, who was riding next to him. He then quickly ordered the halt of the caravan in astonishment of all his companions and followers. To stop in such a place under the hot blazing midday sun, at such a desolate and arid place in the desert called Ghadir Khum.

The 18th of Zill Hijja marks the auspicious occasion of Eid-e-Ghadir. This is the day of nomination of Imam Ali (as) as the successor to the Holy Prophet (pbuh) and commander of the faithful and the first Imam. This is also the day when the famous verse Ayat e Ikmaal " [Today we completed the DEEN for you and finalized the NAIMAT for you] announcing the completion of the DEEN and the finalization of the NAIMAT (blessings) was sent.
This is a highly virtuous day about which Imam Ali Reza (as) has said that "On the day of judgement four days will be presented before Allah (swt) with such glamour as that of a bride - Day of Friday, Eid-ul-Fitr, Eid-ul-Azha and Eid-e-Ghadir - and the day of Eid-e-Ghadir will be the most distinct and will be

1. It is recommended to take bath early in the day for purification.
2. Observe fast on this day.
3. Recite Ziarat-e-Ameenullah and Ziarat-e-Mutliqah
4. Recite Dua-e-Nudba
5. Offer 2 rak'at prayer of Eid-e-Ghadir before mid-day. In each Rak'at after Surah Hamd recite Surah Tauheed 10 times, Surah Qadr 10 times and Ayat al-Kursi 10 times. This prayer is highly rewarding and whatever dua is asked from Allah (swt), inshallah, will be granted. It is recommended to recite this prayer closer to but before mid-day since that was the time when Prophet Muhammad (pbuh) announced the successorship and nomination of Imam Ali (as) as master of the faithful.

As to the Righteous, they shall drink of a Cup (of Wine) mixed with Kafur.
A Fountain where the Devotees of Allah do drink, making it flow in unstinted abundance.
They perform (their) vows, and they fear a Day whose evil flies far and wide.
And they feed, for the love of Allah, the indigent, the orphan, and the captive, (Saying), We feed you for the sake of Allah alone: no reward do we desire from you, nor thanks.
"We only fear a Day of distressful Wrath from the side of our Lord."