Mark 2:23-27
Jesus says in the end “The Sabbath was made for man, not man for the Sabbath. 28 So the Son of Man is Lord even of the Sabbath.”
Jesus often ran afoul of Sabbath traditions, such as when He healed the sick or His disciples picked heads of grain. On the one hand, Jesus was a little loose with observance of the Sabbath. Yet he does insist on observance of the Sabbath, as you have pointed out.
And with this I think we can see that it comes down to what he was pointing out to the Pharisees, which is that they observed the letter of the law rather than the spirit of the law.
Looking at Exodus 20:8-11, it is explicit that Sabbath is a day of rest after having worked six days. No more than that.
Therefore, observing Sabbath on Saturday or Sunday is one of those situations of observing the letter of the law versus the spirit of the law. Jesus was all about the spirit of the law such that it should be compassionate and honors God. During His lifetime, Jesus remained Jewish and observed the traditions. But He also was more liberal in interpreting the traditions so that they fulfilled the intent rather than serve as ammunition for the Karens of those days, the Pharisees.
We can't read too much beyond the purpose of the Sabbath than Genesis 2:2-3 and Exodus 20:8-11. Anything that was added later is set straight by Jesus in Mark 2. One ought to take the word of Jesus that Sabbath is a required holy day to chill. The overriding requirements, in my opinion, are that we rest every seven days and honor God for his creation. Sabbath is a gift from God, not a job from God. And if Jesus is cool with small, compassionate variances on the observance of Sabbath here and there, then we can be sure it's OK with God.
Whether it's Saturday or Sunday, you're still resting every seventh day and honoring God for his creation.
That made no sense to me if we are observing the letter of the law vs the spirit of the law...
That argument always seemed off to me, just because we have no problem with the letter of each of there other 9 ten commandments. Just the 'spirit' of the fourth?
Doing good on the Sabbath was never the issue. The Pharisees made it a burden. It's not and was never intended to be such.
That's not what scripture says though, we are changing the bible if we say 'it doesnt matter which day'. The commandment is to observe the seventh day. Not the first. Scripture is very clear which is the Sabbath.