Those who do not have a traditional Jewish background could be confused by this. There are two possibilities, none of which relate to Sunday as a day of worship.
The phrase “first day of the week” is not found in the Greek. It actually says, “the first of Sabbaths.” This could refer to the seven week “counting of weeks” or the “counting of the Omer.” This is the forty-nine day count between the first night of Unleavened Bread until Pentacost.
Secondly, the Jewish practice after the seventh day Sabbath is to assemble for havdalah (separation) and break bread. Traditionally, after the lighting of the havdalah candle, the drinking of wine, and the smelling of spices, those assembled wish each other a good week (shavuah tov). This is then officially the first day of the Sabbath (week).
Although there is a lot to be said for the “first of Sabbaths” to be referring to the counting of the Omer (in which case, this event could have happened on any day of the week), the second explanation fits the context best. They met after conclusion of the seventh day Sabbath, Saturday evening, in a traditional havdalah service, and Paul spoke until after midnight, to leave Sunday morning for travel.
Regardless of any of the views, the fact that this meeting occurred is in no way proof that they met each Sunday.
Thank you! On this 30th Day of the Omer.