At that time Jesus was going through a field of grain on the sabbath. His disciples were hungry and began to pick the heads of grain and eat them. When the Pharisees saw this, they said to him, "See, your disciples are doing what is unlawful to do on the sabbath." He said to them, "Have you not read what David did when he and his companions were hungry, how he went into the house of God and ate the bread of offering, which neither he nor his companions but only the priests could lawfully eat? Or have you not read in the law that on the sabbath the priests serving in the temple violate the sabbath and are innocent? I say to you, something greater than the temple is here. If you knew what this meant, 'I desire mercy, not sacrifice,' you would not have condemned these innocent men. For the Son of Man is Lord of the sabbath." (Mt 12:1-8)
Sabbath law was something sacred for the Jews and they observed it with strictness. They also made sure that everyone observed it. Sabbath rule also received its sacredness in the way it was attached to creation stories. God rested from creative works on the Sabbath so every Jew should abstain from activities that were creative in nature (like: lighting a lamp is creating light, preparing food is also creative etc).
The story of David in 1 Sam 21:1-6 speaks about anointed king David on the run from Saul who was an enthroned king. Since David was anointed a king, the priests were supportive of his Royal claim and hence they were willing to give him the holy bread. Because of the priestly ministry, the priests were innocent of the breaking of the Sabbath law.
Jesus puts forward his claim to be the king and secondly he is greater than the temple. However his claim to be lord of the Sabbath is quite strong but his appeal for mercy is what counts and in fact counter acts. Mercy and humanity have greater claims than the temple and temple practices.
