Matthew 6:17,18 - Coke's Commentary on the Holy Bible

Anoint thine head— That is to say, "Affect nothing which is uncommon; and, rather than put on a sad countenance, which may shew to all around you that you fast, wash, on the contrary, your face, and anoint your head." Except in times of deep mourning, or public fasting, when they used dust and ashes, which must have sadly deformed the countenance, the Jews were accustomed to wash and rub themselves with oil,which was commonly perfumed, especially on festivals. See Ruth 3:3. Ruth 3:2 Sam. 25: 2. Luke 7:37-38. Such were our Lord's directions to his disciples with respect to fasting, from which it appears that he approved of the duty; and indeed the usefulness of it is sufficiently evident; for by abstinence from food, the body is mortified and subjected to the spirit, and the spirit itself is better fitted for the exercises of devotion: nevertheless, in religious fasting, regard must be had to men's constitutions; for it may happen to some that a total abstinence from food would, insteadoffittingthem for the exercises of piety, render them wholly incapable thereof; in which case no more than a due degree of abstemiousness should be practised. See Macknight, Fortuita Sacra, p. 18 and Explication de Textes difficiles, &c.