Imam al-Sadiq (a) said,
the day of Ghadir Khumm is the grand eid of God. God has not sent any prophet unless he has celebrated this eid, and has glorified it. And its name in the heavens is the day of covenant and on the earth is the day of solemn covenant and public presence.
Ḥurr al-ʿĀmilī, Wasāʾil al-Shīʿa, vol.8, p.89.