But thou when thou fastest, anoint - That is, appear as you do daily. Do not assume any new appearance, or change your visage or dress. The Jews and all neighboring nations were much in the habit of washing and anointing their bodies. This washing was performed at every meal; and where it could be effected, the head, or other parts of the body, was daily anointed with sweet or olive oil. In a warm climate, exposed to the great heat of the sun, this practice conduced much to health, preserved the skin smooth and tender, and afforded a most grateful sensation and odor. See Mark 7:2-3; James 5:14; Mark 11:13; John 12:3.
The meaning of this whole commandment is, when you regard it to be your duty to fast, do it as a thing expressing deep feeling or sorrow for sin, not by assuming unfelt gravity and moroseness, but in your ordinary dress and appearance; not to attract attention, but as an expression of feeling toward God, and he will approve and reward it.

Anoint thine head and wash thy face - These were forbidden in the Jewish canon on days of fasting and humiliation; and hypocrites availed themselves of this ordinance, that they might appear to fast. Our Lord, therefore, cautions us against this: as if he had said, Affect nothing - dress in thy ordinary manner, and let the whole of thy deportment prove that thou desirest to recommend my soul to God, and not thy face to men. That factitious mourning, which consists in putting on black clothes, crapes, etc., is utterly inconsistent with the simplicity of the Gospel of Christ; and if practised in reference to spiritual matters, is certainly forbidden here: but sin is so common, and so boldly persisted in, that not even a crape is put on, as an evidence of deploring its influence, or of sorrow for having committed it.

But thou, when thou fastest,.... Christ allows of fasting, but what is of a quite different kind from that of the Jews; which lay not in an outward abstinence from food, and other conveniences of life, and refreshments of nature; but in an abstinence from sin, in acknowledgment and confession of it; and in the exercise of faith and hope in God, as a God pardoning iniquity, transgression and sin; wherefore cheerfulness, and a free use of the creatures, without an abuse of them, best became such persons.
Anoint thine head, and wash thy face; directly contrary to the Jewish canons, which forbid these things, with others, on fast days:
"On the day of atonement, (say (i) they,) a man is forbidden eating and drinking, "and washing and anointing", and putting on of shoes, and the use of the bed.''
And the same were forbidden on other fasts: in anointings, the head was anointed first, and this rule and reason are given for it:
"he that would anoint his whole body, , "let him anoint his head first", because it is king over all its members (k).''
Anointing and washing were signs of cheerfulness and joy; see Ruth 3:3.
(i) Misn. Yoma, c. 8. sect. 1. & Taanith, c. 1. sect. 4, 5, 6. T. Bab. Yoma, fol. 77. 2. Taanith, fol. 12. 2. Moses Kotsensis Mitzvot Tora, pr. affirm. 32. (k) T. Bab. Sabbat, fol. 61. 1.

But thou, when thou fastest, anoint thine head, and wash thy face--as the Jews did, except when mourning (Daniel 10:3); so that the meaning is, "Appear as usual"--appear so as to attract no notice.

Anoint thy head - So the Jews frequently did. Dress thyself as usual.