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.
Who mentioned legalism? As unsettling as it is to most Christians, Jesus was a Jew. He observed appropriate ordinances and discarded those that were absurd. He observed the 7th day Sabbath. He is our example. The disciples observed it as well. Harvest and moon phase sabbaths did indeed fall in midweek. Those that coincided with the 4th Commandment Sabbath were called "high sabbaths". The Lord was over the Sabbath and therefore could interpret what was acceptable (unlike the Pharisees or even most of todays spiritual leaders). Healing was, of course, most acceptable. Looking back, one wonders why was this even an issue. That was legalism and plain wrong.