It was the Sabbath (or Saturday) and Jesus' disciples were hungry. As they passed through the fields, they plucked the heads of grain (probably barley) and began to eat. The Pharisees witnessed this and objected. They didn't believe that the apostles were guilty of stealing grain (cf. Deut. 23:25). They did object, however, to the fact that the disciples were plucking grain. The Pharisees considered this to be a form of reaping. Thus, it was "work" that was prohibited on the Sabbath day (Exo. 20:8-11).
The fact that the Pharisees choose to object to Jesus' disciples eating in the fields shows the sinlessness of Jesus in a strong way. They hated Jesus with a passion, yet they attack Him here on what seems to be a trivial issue. Evidently, they could not find much to complain about regarding Jesus--otherwise they wouldn't have wasted time with this issue. Let it be remembered that Jesus never committed sin (Heb. 4:15) and neither did He condone sin in others. He "came into the world to save sinners" (I Tim. 1:15)--not to encourage sin. Thus, it is certain that Jesus' disciples did not commit sin here. He would not have allowed them to sin in His presence without giving correction. His apostles did not violate the law on this occasion, but they did violate one of the Pharisees' traditions!
Jesus responds to them in Matthew 12:3 with a bit of irony - "Have you not read...?" These men prided themselves on their knowledge of the Scriptures and their traditions. They were quite familiar with David and his actions that were recorded in I Samuel 21:1-6.
In that context, David ate the showbread which was only lawful for the priests to eat (cf. Lev. 24:5-9). Clearly, David sinned in doing this, and Jesus affirmed such in Matthew 12:4. It "was not lawful" for David to eat the showbread. What does this have to do with the present objection, however? How is this related to the accusation the Pharisees have brought against our Lord's apostles? Was Jesus acknowledging that His followers were sinning (like David) by plucking the heads of grain to eat? Absolutely not, for the reasons mentioned earlier.
In essence, I believe Jesus' argument here is this: You Pharisees justify David's actions when he was hungry even though he committed sin, yet you condemn My disciples for satisfying their hunger by doing that which the law does not condemn! It would seem that Jesus, in referencing David's actions, is attempting to expose the inconsistency of the Pharisees.
We will have more to say regarding this difficult passage tomorrow.