In Matthew 12, a dispute arises between Jesus and the Pharisees regarding the proper observance of the Sabbath. Jesus seeks to follow the original intent and spirit behind the Sabbath Law; the Pharisees are more interested in enforcing the Sabbath traditions set up by men.
For instance, when Jesus' disciples get hungry and pick some grain from the fields to eat, the Pharisees accuse them of breaking the Sabbath by harvesting. To refute that charge, Jesus defends the disciples by citing some Old Testament examples of times when Sabbath interpretations were broken because of the greater needs of the moment.
When a man with a withered hand enters the synagogue on a Sabbath, the Pharisees take the position that healing is work and thus forbidden. But Jesus points out that healing is God's work, and that good things should take place and be celebrated, especially on the Sabbath! Just like rescuing an animal is permissible on the Sabbath, so is doing something good for a human being.
Don't underestimate how sharp this disagreement was or how important it became. It is after these two incidents that Matthew says the Pharisees "went out and conspired against him, how to destroy him" (vs. 14).