It is a sad fact that corruption began in the early church as it lost its roots. As pagans, that worshipped the sun, were incorporated into the church, their day of worship was contemporaneously observe (Sun Day). The truth is Jesus kept the Sabbath as prescribed in the Fourth Commandment. He proclaimed the Sabbath as the Lords Day (Mark 2:28). There is no Biblical commandment to observe Sunday as there is for the sabbath. If we love Jesus He reminds us to keep His Commandments.
The Lord’s Day merited mention in the Bible as a day of observation. It takes some twisting to attempt to reason from that to the purported conclusion that observing it is wrong. In parallel to the Old Testament it would be quite fair to call it “a” sabbath, in the sense that various Old Testament observations that fall on weekdays are called sabbaths. This one is different, however, in that it’s voluntary. Legalism about it is just plain wrong.