Deuteronomy 5 (see also Ex 20) lays out the Ten Commandments, and the fourth commandment has to do with observing the Sabbath. It's rather long since it includes all of the kinds of people and things that are not to be employed for work - in other words, don't try anything weasel-like to try to profit from the Sabbath! Then the question is: what is work? That's where interpreters of the Torah-Law come in.
And that brings us to the end of Mark 2 when Jesus gets challenged by the Pharisees about the failure of his disciples to observe the Sabbath prohibition on work. The disciples had plucked grain from a wheat field while passing through. Jesus' response seems odd; it doesn't refer to the Sabbath at all.
It doesn't get any better in the next story when Jesus is about to heal someone on the Sabbath. Before acting, he poses his own legal question to the Pharisees. But this question also seems to have nothing to do with the Sabbath. What is Jesus' point in these tactics?
Watch the sermon video to see why Jesus is being so roundabout as he talks about the Kingdom while the Pharisees talk about religion.