When was the first namaz (stated prayer) of the Festival of Sacrifices performed?

The Noble Prophet (pbuh) arrived to Medina from the battle of Sewiq on the ninth of the month of Zilkhijja. The following day, the Prophet (pbuh) went to the open-air prayer place along with the other Muslims. He and other Muslims performed two rakah (a series of ritual movements form part of the namaz) of worship without Adhan (call to prayer). He delivered a sermon following the prayer. In the sermon he delivered, he ordered Muslims to offer sacrifices to God Almighty. He himself sacrificed two animals.

While sacrificing his white ram that he bought, he said, O my Lord! I sacrifice this animal on behalf of all of my ummah (Muslim community) who testify your Oneness and what have come to me from you.

During the second sacrifice, he said, O my Lord! This is for Muhammad (pbuh) and his household. He, his family, and the poor ate it. (1)
This is how the first of the Festival of Sacrifices was performed in Islam.

1. Seerah, 3/58-59; Tabaqat, 2/33