Jesus is the Lord of the Sabbath, He alone has the authority to interpret the law. He is saying that He is sovereign ruler or master over the Sabbath day. He is the master of this day for He as Creator made it (Ge 2:1-3, cf Col 1:16+).Their violent response and desire to destroy Him (Lk 6:11+, cf Mt 12:14+, Mark 3:6+)