Albert Barnes Commentary Matthew 6:17

"But thou, when thou fastest, anoint thy head, and wash thy face;" — Matthew 6:17 (ASV)

But you, when you fast, anoint, etc. That is, appear as you do daily. Do not assume any new appearance, or change your countenance 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 done, the head or other parts of the body were daily anointed with sweet or olive oil. In a warm climate, exposed to the great heat of the sun, this practice contributed greatly to health, preserved the skin smooth and tender, and provided a very pleasant sensation and fragrance. See Mark 7:2-3; James 5:14.

Mark 6:13; John 12:3. The meaning of this whole commandment is, when you consider it your duty to fast, do it as an expression of 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 towards God, and He will approve and reward it.