first day
It was the breaking of bread for which the disciples were assembled. The passage indicates the use by the apostolic churches of the first day, not the seventh.
Compare (1 Corinthians 16:2).
Related Commentaries of Acts 20:7
Acts 20:7
7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.