Salaam dear young friends, and heartiest felicitations to you. Can you guess the reason for this congratulations?
Today is, of course, a day of celebration. The 15th of Zil-Hijjah is the birthday of Imam Ali an-Naqi al-Hadi (AS), the 10th Infallible Heir of the Prophet Muhammad (SAWA). It is indeed an auspicious event, but it is not the occasion for which we are greeting you in the warmest possible manner.
So what is it?
Yes, you are absolutely right. We are on the threshold of the Grandest Eid. It is the day on which God Almighty perfected faith, completed His blessings, and decreed Islam as the religion of all mankind (Holy Qur’an 5:3).
It is the 18th of Zil-Hijja. It is the “Day of Ghadeer”. It is the greatest ever day in the annals of human history and civilization.
It was on this day, while returning from his Farewell Hajj Pilgrimage that the Almighty’s Last and Greatest Messenger, on receiving an express commandment from the heavens, halted at the place called Khom, beside a pond (Ghadeer), near Juhfa, and after delivering a memorable sermon to a huge gathering of 120,000 Muslims, proclaimed his dear cousin and son-in-law, Imam Ali ibn Abi Taleb (AS), as the divinely-decreed vicegerent, with those immortal words: “For whomever I am Master, this Ali is his/her Master.”
Prophet Mohammad (SAWA) then supplicated to Lord Most High: O Allah! Befriend whoever is a friend of his, and turn away whoever is his enemy; assist whoever helps him and humiliate whoever tries to humiliate him.
No wonder, Ghadeer is supposed to be the greatest Eid in Islam. As a matter of fact, according to the Prophet’s 6th Infallible Heir, Imam Ja’far as-Sadeq (AS), it is the Greatest Eid in Islam, more significant than Eid al-Fitr and Eid al-Adha, because this day decided the continuation of divine guidance for humanity, despite the plots of the seditious, such as the scandalous gathering of Saqifa Bani Sa’da.
It is the day on which God Almighty perfected faith, completed divine favours, and decreed Islam as the religion of mankind. 
May God increase in the hearts of you youngsters – as well as in our hearts – the love, affection, and veneration of the Commander of the Faithful, the Leader of the Pious, the Symbol of Justice, the Epitome of Wisdom; the Paragon of Valour that the Prophet’s First Infallible Heir was! Amen.