NOTES for Mat 12:1-21
On the holy day of the Sabbath, the law permits no work. This day belongs to God. On the Sabbath a devout Jew must attend the synagogue and listen there to the Torah, the Law, God's revelation given to Israel, or read it himself. The only ones who must practice their professional work on that day are the priests and Levites in the Temple, because they do not work for themselves but perform service to God: they circumcise and offer sacrifices. This is precisely the argument Christ gives the Pharisees in the Gospel of Matthew. For He is the One greater than the Temple. His whole life is continuous worship, the fulfillment of the Father's will. He does His will always, including on the Sabbath, just as the daily sacrifice in the Temple must not be interrupted for the Sabbath.
The same applies to us: through faith in Christ we become a "royal priesthood." Our whole life is transformed into service to God, carried out on Saturday and Sunday and at every moment of time. All our deeds must be dedicated to the Lord, and every thought or word of ours must be a pleasing sacrifice to God, one that does not cease for even an instant.