OK he literally calls it "the day called that of the sun" in Greek. No bones about it, he is talking about sunday, the day after the sabbath of the Jews.
200 years before Constantine the church fathers were without controversy saying that Christian tradition was to meet in the 8th day / 1st day and also adamantly affirming that this was the day Christ rose. This is one generation from the apostles themselves, people who sat at the feet of the apostles.
'blah blah Constantine changed the day Christians met' is rubbish. These people were being killed just for believing in Christ, you think they would go along with being forced to worship on a different day? Nope.
All that said, sunday is not sabbath. Christians don't meet on sabbath, that was never a command given to us - were not observing sabbath when we meet; that's not the purpose.
and anyone calling the 8th day 'Christian sabbath' is every bit as confused as the SDA.