I don't believe anyone has said that Acts 20:7-9 takes place on the Sabbath (seventh) day.
The last day was from sundown Friday to sundown Saturday.
And the first day was from sundown Saturday to sundown Sunday. Acts 20:7 is taking place on the "first of the week" right after sundown on the Sabbath! If you look at the Greek you will see that the word "DAY" has been added. It is simply the "First of the Week" and this would be just another way to say Saturday night......shortly after Sabbath services had ended.
This is a "Havdalah Meal".....an after Sabbath fellowship as we have all said.
Thus says the rabbis. As I have poined out before, it does not say, they were gathered for the Jewish last day sabbath and then hung around for a Jewish fellowship meal. It says, "The came together on the first day of the week to break bread," etc. There is absolutely no mention of any last day sabbath worship in the context. All the worship happened entirely on the first day of the week.