Hi, brother Yeghian here in Alaska. The oldest known extant "church bulletin" is the Didache, or rather, HDIDAXH, and in the Medieval and Modern Script, η διδαχη. We date this to the mid to mid late 1st century, which makes it right back there with the oldest fragment of Mark. When folks say that we don't have 1st century Christian documents just smile and say, "Well, my Christian brother Yeghian has copies of 1st century Christian documents all over his house. He can't play football or basket ball, and bass boats aren't his thing, so old sheets of papyri is kind of this thing."
When we read the Didache it is a magical window into Christianity immediately just after Paul sowed the seeds. Remember, - Paul got chucked out of Jerusalem, and he went out into what is in Hebrew "The Nations." Our brothers and sisters in the 1st century were first from Asia Minor. That means they were either Jews or Greeks. Now, - the Jews probably followed their customs, which if you haven't read Galatians slowly and carefully, you might miss what Paul is saying.
As for the Greeks, these early brothers and sisters from their Pagan traditions of going to "church" on a "sabbath." That wasn't introduced until 321 AD when Constantine issued his "Dies Solis." Step back and take a look at who there is really be celebrated. Constantine grew up worshiping "Apollo," and Apollo was the god of what?
So, fast track it back to who Christians were and what they were doing in 70-90 AD in Asia Minor, read the Didache and notice that is perfectly matches what Paul taught about Christians banging out a living with our hands and sweat.
What this means is this. In Asia Minor, we worked to Sun Down and were ready for work before the Sun came up. See Mark 4:26-29 and read between the lines. Whom was Jesus talking to? People who had regular days off? In the Roman world there was no such thing as a regular day off before 321 AD.
70 - to 125 AD as an example, we met after slave hours and our masters let us. We shared food, pulled splinters out of each other fingers, put olive oil on our backs with herbs mixed in to keep the scars from getting infected. Consequently, when we read the Didache, there is absolutely no reference to a set "day." No such concept as a "Seven Day" week in the Didache. In Acts, if you are Jewish, that is up to you, but as a member of the "Nations," what both Jesus and Paul call the ΕΘΝΟΙ, εθνοι, or in Roman letters, ethnoi, (Acts 21:19 as an example) we met when our masters let us. That was after all the family went to sleep.
Here is a link to the Didache. Notice the writer records the rules, but makes absolutely what "Day" they met or must meet, and you can match that with Acts. It is amazing because it lines up perfectly with Paul's "tent" business. The day of the week for "church" was made law in 321 AD.
According the Acts and the Didache, the right day of the week is "today."
What is the Didache? https://www.ancient.eu/Didache/
Isn't English translation isn't "bad," but it is typically "stiff": https://carm.org/didache
You are a blessing, thank you.