At the beginning of John's Apocalypse, the Revelation of Jesus Christ (Rev 1:1), John says, "I, John, your brother and fellow partaker in the tribulation and kingdom and perseverance which are in Jesus, was on the island called Patmos because of the word of God and the testimony of Jesus. I was in the Spirit on the Lord's day, and I heard behind me a loud voice ..." (Rev 1:9-10). Interesting phrase, there: "on the Lord's day".
From the beginning, for Christians, Sunday has been the Lord's day. That is, it has been the day that Christians gather to celebrate with each other that very first "Lord's day" when the women went to the tomb and found it empty, when the disciples heard He had risen and ran to see, when Jesus spoke to the disciples on the road to Emmaeus and told them how the Scriptures pointed to Him. It was on that "first day of the week" that Jesus breathed on them and said, "Receive the Holy Spirit." (John 20:19-23). Believers gathered on the first day of the week (Acts 20:7; 1 Cor 16:2). It was the special day for believers. Even in my lifetime "the Lord's day" was a phrase that was always intended as a reference to Sunday and no other.
When, do you suppose, that changed? It used to be that Christians felt compelled to be in church on Sundays. These days, not so much. In former times even "less-than" believers felt compelled to be in church on Sundays. Now Christians feel like it's a private thing and maybe it's in church with other believers and maybe it's not. Maybe it's all by yourself in a forest ... or your family room ... or wherever. "It's a personal thing," they, often belligerently, claim. "God is wherever I am, so it doesn't matter if I go to church." Sounds spiritual, doesn't it? And if they do go to church, they prefer one that is "all about me", so to speak. They want it to be "interesting" and "relevant" and, by all means, "contemporary". None of that "old time religion." Upbeat, catchy, get my feet moving and move me. You know, get me feeling warmly toward God. So they pick up the music and limit that preaching stuff and when they do preach it has to be relevant and applicable and personal, not all that doctrine and theology stuff. Because it is no longer "the Lord's day". When did that happen?
I don't know the answer. I just know it did. People might go to a good church with good preaching and all, but they're still mostly fixated on self. "What did you get out of church?" isn't an uncommon question. Because it isn't about the Lord; it's about me. Even good pastors find their hands tied because even good church members don't really want to go too deep. Dumb it down. We want to feel good about God. And we don't want you to go alienating people, pastor.
It's still the Lord's day because it's still the day that commemorates His resurrection. I'm just not clear anymore how many actually see it that way. It is supposed to be all about the Lord. It's not as much anymore. Too bad. At least, too bad for God. Probably for us believers, too.