Mark 2:18

Christ had been put to justify himself in conversing with publicans and sinners: here he is put to justify his disciples; and in what they do according to his will he will justify them, and bear them out. I. He justifies them in their not fasting, which was turned to their reproach by the Pharisees. Why do the Pharisees and the disciples of John fast? They used to fast, the Pharisees fasted twice in the week (Luk 18:12), and probably the disciples of John did so too; and, it should seem, this very day, when Christ and his disciples were feasting in Levi's house, was their fast-day, for the word is nēsteuousi - they do fast, or are fasting, which aggravated the offence. Thus apt are strict professors to make their own practice a standard, and to censure and condemn all that do not fully come up to it. They invidiously suggest that if Christ went among sinners to do them good, as he had pleaded, yet the disciples went to indulge their appetites, for they never knew what it was to fast, or to deny themselves. Note, Ill-will always suspects the worst. Two things Christ pleads in excuse of his disciples not fasting. 1. That these were easy days with them, and fasting was not so seasonable now as it would be hereafter, Mar 2:19, Mar 2:20. There is a time for all things.

— Matthew Henry's Concise Commentary (public domain)