Therefore the Son of man - See the notes at Matthew 12:8.

The Son of man is Lord - See on Matthew 12:7, Matthew 12:8 (note). Some have understood this as applying to men in general, and not to Christ. The Son of man, any man is Lord of the Sabbath; i.e. it was made for him, for his ease, comfort, and use, and to these purposes he is to apply it. But this is a very harsh, and at the same time a very lax, mode of interpretation; for it seems to say that a man may make what use he pleases of the Sabbath; and, were this true, the moral obligation of the Sabbath would soon be annihilated.
God ordained the Sabbath not only to be a type of that rest which remains for the people of God, but to be also a mean of promoting the welfare of men in general.
The ordinances of religion should be regulated according to their end, which is the honor of God, and the salvation of men. It is the property of the true religion to contain nothing in it but what is beneficial to man. Hereby God plainly shows that it is neither out of indigence or interest that he requires men to worship and obey him; but only out of goodness, and to make them happy. God prohibited work on the Sabbath day, lest servants should be oppressed by their masters, that the laboring beasts might have necessary rest, and that men might have a proper opportunity to attend upon his ordinances, and get their souls saved. To the Sabbath, under God, we owe much of what is requisite and necessary as well for the body as the soul.

Therefore the Son of man is Lord also of the (k) sabbath.
(k) Has the sabbath day in his power, and may rule it as he desires.

Therefore the son of man is Lord also of the sabbath. Meaning himself, who had a power not only to dispense with it, but to abrogate it as he did, with the rest of the rituals of the ceremonial law; See Gill on Matthew 12:8. So that it did not become them to find fault with what his disciples did, with his leave and approbation.

The Son of man is Lord also of the sabbath. This affirms the Divine nature of Jesus. None but the Divine is Lord of a divine institution. Since Christ is Lord of the Sabbath day, he has the right to modify it, to adapt it to the new dispensation, and to change the time of its observance from the last day of the week to the first, so as to make it the memorial of the beginning of the New Creation, instead of commemorating the rest from the first creation.

Moreover the Son of man is Lord even of the Sabbath - Being the supreme Lawgiver, he hath power to dispense with his own laws; and with this in particular.