Saturday evening is the close of sabbath but it is more likely that Paul would have been teaching on Saturday evening instead of on Sunday evening. Why is this so? Because the verse itself states that he kept on talking until midnight because he intend to leave the next day which would be Sunday. Thus Paul, would have been meeting on the sabbath as was his usual habit and kept on teaching past sundown because of his planned departure. That is even more evident when one examines Paul's habit of meeting in the chapters leading up to Acts 20.
Acts 13:14 “14 From Perga they went on to Pisidian Antioch. On the Sabbath they entered the synagogue and sat down….”
Acts 14:1 “At Iconium Paul and Barnabas went as usual into the Jewish synagogue. There they spoke so effectively that a great number of Jews and Greeks believed”
Acts 16:12 From there we traveled to Philippi, a Roman colony and the leading city of that district of Macedonia. And we stayed there several days.13 On the Sabbath we went outside the city gate to the river, where we expected to find a place of prayer.
Acts 17:1-2: “When Paul and his companions had passed through Amphipolis and Apollonia, they came to Thessalonica, where there was a Jewish synagogue. 2 As was his custom, Paul went into the synagogue, and on three Sabbath days he reasoned with them from the Scriptures…”
As far as I'm aware, there's no scriptural record of Paul meeting on Sundays so I have no good reason to believe otherwise.
Except Paul tells us the day he was speaking using 1st century jewish customs of numbering days saying "one of the Sabbath" (mia ton sabbaton) and this means Sunday. This is a what the overwhelmingly majority of scholars agree on.
in 59 english translations there are as follows:
47 "first day of the week" (YLT uses "first of the week")
[KJ21, ASV, AMP, AMPC, BRG, CSB, CEB, SEC, DARBY, DLNT, DRA, EHV, ESV, ESVUK, EXB, GNV, HCSB, ICB, ISV, PHILLIPS, KJV, AKJV, LEB, MEV, MOUNCE, NABRE, NASB, NCV, NET, NIRV, NIV, NIVUK, NKJV, NLV, NLT, NRSV, NRSVA, NRSVACE, NRSVCE, NTE, RSV, RSVCE, TLV, WEB, WE, WYC, YLT]
7 Sundays (not including AEB, EXB which has sunday in parentheses and listed above, the EXB even goes so far to suggest Sunday night)
[ERV, GW, TLB, MSG, NOG, TPT, VOICE]
1 Yom Rishon (which is Sunday)
[OJB]
1 "first of the sabbaths"
[JUB]
1 "morrow after the Sabbath day"
[NMB]
1 Saturday evening
[GNT]
1 Motza’ei-Shabbat (which is Saturday evening)
[CJB]
Not one translation states that this gathering was on the Sabbath and the closest would be Motza’ei-Shabbat which is the time in the evening immediately following the Sabbath but technically a different day. Jews counted days sundown to sundown, where Greeks counted them by the morning. It is unclear what actually is meant here and it is possible it is Saturday evening which technically is the start of first day of the week or it could be Sunday evening depending on how the author is counting days but what is clear is that it is the day after the Sabbath and that the customer was to gather on this day to break bread.
We count our days differently and it is possible the value of when this gathering starts has shifted from saturday evening to sunday (morning) but it still captures the value of meeting on the first day of the week and that value was never on the Sabbath but after the Sabbath. This is probably so as not to disturb customs of practicing Jews that were believers.
