Sir,
Could you please translate this from the Greek for us?
It is from the Didache of the 12 Apostles, dates from around 50 AD. It almost certainly pre-dates the Gospel of Matthew, and may very well be the written record of the Apostles from the Council of Jerusalem, recorded in Acts.
It lays down the ORIGINAL traditions of the Church, as practiced by the Apostles THEMSELVES, so there is no question of interpretations and the interpellations of later generations.
If one wants to discover what the ORIGINAL practices of the apostolic Church were, one reads the Didache.
Section 14:1 of the Didache, reproduced from the Greek below, relates to the Lord's Day, in the words of Peter and the Apostles.
It would, perhaps, illuminate the discussion to see what the men who were at the Resurrection and Pentecost thought about The Lord's Day.
14:1 kata kuriakhn de kuriou sunacqentev klasate arton kai eucaristhsate, proexomologhsamenoi ta paraptwmata umwn, opwv kaqara h qusia umwn h.
Very literally:
"Now/but according to the Lord's of the Lord, having gathered together, break bread and give thanks, having confessed your sins, in order that your sacrifice may be pure."
The key phrase here, of course, is kyriake kyriou, "the Lord's of the Lord." (The "n" at the end of kyriake is simply the accusative ending, since the preposition kata takes the accusative.) "The Lord's ____" . . . what?
The adjective kyriake here is in the feminine, meaning that the noun to be understood would be in the feminine also. The feminine noun that would most naturally fit and be readily understood is probably hemera, "day," which already was becoming a common expression, as in Rev. 1:10.
So the passage from the Didache indicates that very early on (I've not heard a date theorized as early as yours, but still very early), the church customarily gathered on a day known as "the Lord's Day."