Acts 13:14-16
14 But when they departed from Perga, they came to Antioch of Pisidia, and went into the synagogue on the Sabbath day, and sat down.
15 And after the reading of the law and the prophets the rulers of the synagogue sent to them, saying, You men and brethren, if you have any word of exhortation for the people, say on.
16 Then Paul stood up, and beckoning with his hand said, Men of Israel, and you who fear God, give audience.
^
first real organized Church Service on the SABBATH DAY!!
14 But when they departed from Perga, they came to Antioch of Pisidia, and went into the synagogue on the Sabbath day, and sat down.
15 And after the reading of the law and the prophets the rulers of the synagogue sent to them, saying, You men and brethren, if you have any word of exhortation for the people, say on.
16 Then Paul stood up, and beckoning with his hand said, Men of Israel, and you who fear God, give audience.
^
first real organized Church Service on the SABBATH DAY!!
Paul in Acts 20 was in a gathering of believers the first day of the week. He obviously didn't think this to be wrong.