Great question @Zak_Carter. I think one might make the case from the NT that as an apostolic pattern/example (as you mention) Sunday gatherings are commanded.
 But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth.  To this he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ.  So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter. (2 Thessalonians 2:13–15, ESV, emphasis added)
Paul and his fellow apostles gave the Thessalonians not just doctrines but also traditions. And when he commands them in verse 15 to hold to these traditions, he’s talking about all the traditions and not just a particular one.
 Now I commend you because you remember me in everything and maintain the traditions even as I delivered them to you. (1 Corinthians 11:2, ESV, emphasis added)
We see not only the same term (traditions) but the same idea here that Paul expects the Corinthians to hold to all the traditions he passed on to them (assumed to be from Jesus, e.g. Galatians 1:12) just as he charges the Thessalonians to.
 If anyone is inclined to be contentious, we have no such practice, nor do the churches of God. (1 Corinthians 11:16, ESV)
 For God is not a God of confusion but of peace. As in all the churches of the saints,  the women should keep silent in the churches. For they are not permitted to speak, but should be in submission, as the Law also says. (1 Corinthians 14:33–34, ESV)
What’s notable in both of these passages is that, though in context Paul is speaking to specific issues at Corinth, he expects all the churches of God to be in conformity of practice/tradition. He doesn’t expect one church to embrace tradition/practice X and another church to reject tradition/practice X (which is far removed from where we stand today). Paul seems to take for granted that if one church embraces tradition/practice X then all churches should and if one church rejects tradition/practice X then all churches should.
 When you come together, it is not the Lord’s supper that you eat.  For in eating, each one goes ahead with his own meal. One goes hungry, another gets drunk.  What! Do you not have houses to eat and drink in? Or do you despise the church of God and humiliate those who have nothing? What shall I say to you? Shall I commend you in this? No, I will not.  For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread…(1 Corinthians 11:20–23, ESV).
In 1 Corinthians 11:2-16, Paul commends the Corinthians with respect to their holding to the tradition of head coverings as he had taught them. But then in 1 Corinthians 11:17-34, he rebukes them for not holding to the tradition of the Lord’s supper as he had taught them (v.23).
The point is that the Lord’s Supper is a tradition that Paul had given to all the churches and this was one of the main reasons (if not the main reason the church came together): to partake of the Lord’s Supper together.
 On the first day of the week, when we were gathered together to break bread, Paul talked with them, intending to depart on the next day, and he prolonged his speech until midnight. (Acts 20:7, ESV)
Many hold that “break bread” is synonymous with partaking of the Lord’s Supper. If this is the case, then it seems undeniable that gathering on Sunday is a tradition/practice that is common to all the churches because that’s what the apostles not only taught them (Revelation 1:10?) but practiced with them.
But I suppose one might argue that they could have gathered to break bread on the second and third day (and every other day of the week) in addition to the first day and it just so happened that this particular day that Luke writes of was the first. This argument would be consistent with Acts 2:46 where the early church is described as breaking bread (same term as in Acts 20:7) in their homes every day (day by day) and not just Sunday.
The point remains that they at least gathered on Sunday based on apostolic tradition which is commanded by God in contrast to human traditions (Matthew 15:1-9).
I guess the operative question then becomes: what did the apostles teach should take place during such gatherings (at least the Lord’s Supper?) and is it what we would call “Sunday worship”?