The day about the revelation of the verse " completion of Islam" must be the day of Eid

Please read the narration a Jew said to Umar bin Khattab "if it had been revealed in relation to us, we would have taken that day as the day of rejoicing."

Umar bin Khattab said I know this verse was revealed at the day of ARAFA.

We all know that day of ARAFA  is already day of Eid among Muslims. There was no point that Jew would tell this thing to Umar bin Khattab that you people are not celebrating it while Muslims were already celebrating EID!

Actually this verse was revealed at the day of Ghadir after the announcement of successorship of Imam Ali(as). This is one of those measures to make people forgetful about Ghadir that this verse was revealed at the day of Arafa.

We know that Shias all around the world celebrate (18 Dhul HAjj) that day as EID because the Holy Prophet(pbuh)  was given the certificate of completion of Islam by Allah (swt). These all happen when the Holy Prophet announced about the Successorship of Imam Ali.

The verse says "Today I have perfected your religion and completed my bounty upon you, and I was satisfied that Islam be your religion." (Quran 5:3) (many Sunni sources testify this also, al-Durr al- Manthur, by al-Hafiz Jalaluddin al-Suyuti, v3, p19, Manaqib, by Khawarazmi al-Hanfi, p80, History of Damascus, Ibn Asakir, v2, p75 etc.).

For more info please read our article Philosophy of Caliphate.

Unfortunately Sunni fellows are purposely kept in the dark about Ghadir, they forgot the event of Ghadir. 

Here we want to link a prominent Sunni scholar Tahirul Qadari says  "I have collected 51 chians of Hadith e Ghadir".

http://sunnah.com/muslim/56/3

completion