Why Sunday
 

see in that the Christians did begin to gather together on Sunday in the New Testament.

[Act 20:7 NKJV] 7 Now on the first [day] of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight.

(labor and traders used to get their gain daily, so this is an actual Christian gathering)
[1Co 16:2 NKJV] 2 On the first [day] of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come.

[Rev 1:10 NKJV] 10 I was in the Spirit on the Lord's Day, and I heard behind me a loud voice, as of a trumpet,

The name "Lord's Day" (different in greek to the "Day of the Lord") began to be used to refer to Sunday. As you can see in the early christian writings. So, it is probably correct to say that John was talking about Sunday.

Pulpit Commentary: The phrase had not yet become common in A.D. , as is shown from St. Paul writing, "on the first of the week" (1 Corinthians 16:2), the usual expression in the Gospels and Acts (Matthew 28:1; Mark 16:2; Luke 24:1; John 20:19; Acts 20:7; comp. Mark 16:9). But from Ignatius onwards, we have a complete chain of evidence that ἡ Κυριακή became the regular Christian name for the first day of the week; and Κυριακή is still the name of Sunday in the Levant. "No longer observing sabbaths, but fashioning their lives after the Lord's day" (Ign., 'Magn.,' 9.). Melito, Bishop of Sardis (A.D. 170), wrote a treatise περί Κυριακῆς (Eusebius, 'Hist. Eccl.,' IV. 26:2). Dionysius of Corinth (A.D. 175), in an epistle to the Romans, mentions that the Church of Corinth is that day keeping the Lord's holy day (Eusebius, 'Hist. Eccl.,' IV. 23:11). Comp. also Clem. Alex., 'Strom.,' VII. 12:98 (p. 377, Potter); Tertull., 'De Con.,' 3. and 'De Idol.,' 14, where Dominicus dies is obviously a translation of Κυριακὴ ἡμέρα; and fragment 7 of the lost works of Irenaeus.

I wonder if the gathering on the first day of the week was because the first Christians were Jews even the ones in exile, and they observed the Sabbath (Friday 6pm to Saturday 6pm), it was natural to keep going (since they didn't work that day) and gather with other Christians at homes after 6pm (Sunday at that point). Hence, Paul teaching until midnight "Paul, ready to depart the next day, spoke to them and continued his message until midnight."

Apparently, it became a day dedicated to seek the Lord together and do the Lord's Supper, that would work for both the Jews keeping the Sabbath and for the Gentiles.

Re: Why Sunday
 

Good question. Was Sunday the beginning of the week then? I don’t see any problem if you want to have service on Saturday morning or night. With that said we are creatures of habit, and tradition so I don’t see churches changing from Sunday to Saturday. Also, the church isn't the building you attend on service times. The church is us, its the bride of Christ, so wherever you go you can have church.

One thing to mention and I’m sure it would’ve come up, that the Old Testaments was Sabbath and we understand all that. Now, New Testament says that the Holy Ghost is our rest we don’t need a physical day to rest as long as were “current” in the Holy Ghost we have rest.

Re: Why Sunday
 

Brother, I am not advocating for Sabbath keeping. I am just curious about New Testament and very early writings about how the gathering on Sundays originated.

The first day of the week in the New Testament started on Saturday evening, at least for the Jews, and it seems to me natural that they still kept the Sabbath to go to the synagogues first and then gather in the evening with the brethren to break bread together and have a service. I may be wrong but I find it interesting, and I have not found any connection like this anywhere.
In addition to that, the resurrection and Pentecost were both on the first day of the week as Hebrews days (Sat 6pm to Sun 6pm).
All of that could have made the first day of the week the day of gathering and seeking the Lord together; later called "The Lord's day".

Re: Why Sunday
 

I take it then you work 7 days a week and likely would require any employees you might have to work 7 days a week, too?

Re: Why Sunday
 

Sunday was the day of the Sun in the Roman Empire. When the Constantine made Christianity the umbrella religion of the empire, he made Sunday the Church day. Why? Because Romans already were use to that being the day of the risen sun.

Re: Why Sunday
 

Pentecost was on a Sunday because the waving of the omer 50 days earlier happened to be on a Sunday that year. Which is why the resurrection happened on the first day of the week: that was the waving of the omer that year. Every year it might fall on a different day of the week, and thus so would Pentecost. This is why Christ was to rise "on the third day", not "on the first day of the week". There is literally zero significance to the first day of the week. Which is why the apostles and early Christians simply called it the first day of the week (while continuing to call the seventh day of the week "the sabbath").

There are no regular gatherings of Christians on the first day of the week recorded in Scripture. The ONE case in Acts was a meeting with Paul who was departing on the morrow (sunday morning), the meeting occurred at the end of the sabbath and went late into the night. Most sabbath keepers (that I know of, anyway) keep a service, however small or informal, at the end of the sabbath into the first day of the week. Jews do something similar. This was the origin of "vespers" or "evening service" by the way.

The day of the Sun god was never called "the Lord's Day" in the Bible, nor by anyone in post apostolic times except the catholic heretics. The only day of the week the Bible identifies as being "of the Lord" is the seventh day, which is the Sabbath, whose Lord is Jesus.

Sunday was sacred to many pagans, it was the day dedicated to the Sun and the sun god. Mithra, Sol Invictus, Amon-Ra, Baal, etc are all names for the sun god in different cultures. It was chosen by the catholic "fathers" because they were bringing in paganism. Constantine made it official. The whole thing is paganism, now with a Christian veneer. As all ancient pagans understood, every deity could go by many names. So Baal nowadays is mostly called "Jesus" by his followers. Another Jesus.

Re: Why Sunday
 

I want to believe you understand what I meant if I wasn't clear enough. My HVAC job I only work 20 hours or less, then rest of my week is for the church.

Re: Why Sunday
 

You said we don't need a day of rest.

Re: Why Sunday
 

20 hours or less? You must be making crazy folding money.

Re: Why Sunday
 

Acts 20:7 looks poorly translated. See here:

https://biblehub.com/interlinear/acts/20-7.htm

Ἐν δὲ τῇ μιᾷ τῶν σαββάτων

En de mia te ton sabbaton

It seems to read better simply as "Then, on one of the sabbaths...".