What does Matthew 12:9 mean?

The Pharisees are eager to catch Jesus doing something unlawful so they can discredit Him. In the previous passage, they confronted Jesus about His disciples plucking and eating heads of grain as they walked through a field on the Sabbath. The Pharisees insisted this was work, harvesting, and a violation of the fourth of the Ten Commandments (Matthew 12:1–8).

Jesus corrected them. Not only was their application of the command too restrictive and unreasonable, it proved they did not understand what God meant in saying that He desires mercy, not sacrifice. Legalists like the Pharisees did not care for the people of God in a merciful way. Even worse, they did not recognize that Jesus was the Messiah and, thus, Lord of the Sabbath.

It's unclear whether catching the disciples plucking grain in the field was a set-up or not. In other words, we're not sure if the Pharisees happened to notice the event, or if they were spying on Jesus' followers. What follows, when Jesus and His disciples reach the synagogue on the Sabbath, has every appearance of an arranged encounter. One way or another, these men are determined to catch Jesus in the act of breaking their interpretation of the Law of Moses.