The Sabbath began on the evening of Friday at sunset and concluded at sunset on Saturday. It was a day reserved to worship God and to rest and no one was permitted to work including slaves and beasts. To break the Sabbath was seen by the Pharisees as a lack of religious commitment.
Jesus along with his disciples are walking through the cornfields eating the corn that they had plucked and rubbed in their hands. The Pharisees regard the action of plucking the corn to be the same as harvesting or gathering the crop and so working. This is clearly a very strict interpretation of the law.
In response to the Pharisees Jesus says to their questioning of his disciple’s actions “so you have not read what David did when he and his followers were hungry”. This would have been an insult to the educated Pharisees, who would clearly have read this in 1 Samuel 21:1. This story speaks of King David entering the House of God and eating the loaves of offering reserved to the priests only.
The point that Jesus is making is that the pressing need of hunger and the fact that King David was King allowed him to over ride the law and so can Jesus, the descendent of King David, the Son of David and Son of God. Jesus is the master of the Sabbath and knows the original purpose and intention of it since it was his Father who gave the original law to Moses “to keep holy the Sabbath day”.
The Pharisees have made the Sabbath into something which was never God’s intention and are manipulating it to suit their and not God’s purposes.