Sorry I wasn't clear.
The apostles "taught the Church about the death, burial, and resurrection of Jesus Christ. The Church first observed His death on the 14th of Nisan on whatever day of the week it fell on. "
Since the apostles first taught that the death was on the 14th of Nisan, then the third day resurrection was on the 16th (14, 15, 16). Since they observed the 14th on whatever day of the week it fell, then the 16th (day of resurrection) also would have fallen on whatever day of the week.
See? The day of death was the 14th and they observed it on whatever day of the week it fell. Same with the resurrection. It was the 16th and they observed it on whatever day of the week it fell. For example, one year the 14th could fall on a Sunday and the 16th on a Tuesday. The next year the 14th could fall on Monday and the 16th on Wednesday. Each day would be the Lord's day.
So the fixing of and definition of and reason for Lord's day as Sunday is a later invention apart from Scripture and Apostolic Tradition. The apostles never fixed or defined or observed the Lord's day as Sunday or Saturday.
Then if we consider all of Scripture, we find this confirmed--
Today is the day, if you hear the LORD, do not harden your heart ...
This is the day that the LORD has made ...