After the resurrection of Jesus, when the old covenant began to pass away from the scene (Heb. 8:13), where did the apostles gather with the people and set aside the last day of the week? Please be specific.
They were keeping the 7th day holy for thousands of years at the specific commandment of God who created the 7th day holy:
Gen 2:3 And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.
You see, God created the 7th day holy at the beginning of the world. He codified it in the ten commandments. Israel and all God fearing non-Israelites kept the day holy. At the time of the apostles there was never a thought, inkling, or idea that it wasn't holy. There was no need to sit down and figure out what day was set aside by God almighty. Everyone knew. It was read in the scriptures every sabbath. Jesus Christ, God almighty on earth, kept it as an example. You would have had to be an illiterate caveman NOT to know that the 7th day was holy and set aside by God.
I would recommend doing a historical study on the evolution of the sabbath in Christianity because history is where it happened, not scripture.