It is “jocor," not "Jacor." Here is my understanding of those references:
John 20:19, 25 - Notice they were "assembled for fear of the Jews," not to worship & fellowship. They were hiding. According to Mk.16:11-14, they didn't believe Yeshua had resurrected so they certainly could not honor that day as the “Lord’s Day” or new Sabbath at that point in time.
The next verse (26) is interesting.
“And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you.
This does not refer to Sunday. Eight days
after a Sunday is a Monday or the start of the third day of the week for the disciples. Why, then, isn’t the Lord’s Day on a Monday? The fact of the matter is, the day the risen Savior appeared to men does NOT determine when the “Lord’s Day” is, nor does it negate the day men are to rest from their labors.
Acts 20:7 - What does it mean to "break bread"? Does it mean to take "the Lord's supper" on Sunday? According to Acts 2:42-46, "breaking bread" was done daily. It simply means to have a meal. In Acts 20:7, the disciples finished
Sabbath services on Saturday. After the sun set ending Sabbath and beginning the first day of the week, they had a meal together followed by Paul's preaching until midnight on the first of the week (our Saturday night). The disciples held this
special gathering because their beloved Paul was to leave the next morning (vs. 7). In the morning, he had planned to walk approx. 10 miles to Assos. Notice he would not take this strenuous, tiring trip on Sabbath, but waited instead until Sunday, another work day.
Rev 1:10 - Is the "Lord's day" a new name for Sunday, a reference to the weekly Sabbath, or a reference to "The Day of the LORD," "The Day of Yahweh"? The latter could be true since John was transported in the spirit to the prophetic time known as "The Day of Yahweh." It seems more likely, however, that John was referring to the weekly Sabbath. Yeshua, in referring to himself, said, "...the Son of Man is Lord also of the Sabbath" (Mk.2:28). In other words, since Yeshua is the Lord or Master of that day, then the weekly Saturday Sabbath is the Lord's day or Yeshua's day. To say that it means Sunday is pure assumption without fact or Scripture to back it up.
However, even if the “Lord’s Day” meant Sunday, that only suggests they honored Yeshua by remembering Sunday as his resurrection day. It does NOT prove they moved the Sabbath from Saturday to Sunday. They could just as easily rested on the Sabbath as usual, but also kept Sunday special in some way.
As for Justin Martyr and Tertullian, they are far removed in time from the practices of the apostolic assemblies. Once the Apostles and the core Jewish believers had died, the predominantly Gentile church began to allow all sorts of false doctrines in. The wolves did not spare the flock as Paul prophesied in Acts 20:29-30. One of those false doctrines was the moving of Sabbath to Sunday. Tertullian wrote, “All anxiety is to abstained from, and business postponed on the Lord’s Day.” De Orat. c. 17. Yet, how many in the churches today do that? Many teach that there is no Sabbath at all, either on Saturday or Sunday.