Matthew 6:16-18 explained

16 Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward. 17 But thou, when thou fastest, anoint thine head, and wash thy face; 18 That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly. (Matthew 6:16-18 KJV)

William Burkitt’s Commentary

The next duty which our Saviour instructs his disciples in is that of religious fasting; which is, a devoting of the whole man, soul, and body, to a solemn and extraordinary attendance upon God, in a particular time set apart for that purpose; in order to the deprecating of his displeasure, and for the supplicating of his favour, accompanied with abstinence from bodily food and sensual delights, and from all secular affairs and worldly business. Now our Saviour’s direction, as to this duty of fasting, is double:

1. He cautions us to beware of an abuse in fasting: Be not as the hypocrites are, of a sad countenance; that is, do not affect a sullen sadness, ghastliness, and unpleasantness of countenance, like the hypocritical Pharisees, who vitiate and discolour their faces, who mar and abolish their native complexion. Hypocrisy can paint the face black and sable, as well as pride with red and white.

2. He counsels us to take the right way in fasting; to anoint the head and wash the face: that is, to look as at other times, using our ordinary garb and attire, and not to affect anything that may make us look like mourners when really we are not so.

Where we may note, That though hypocrites by their dejected countenances and mortified habits do seek to gain an extraordinary reputation for piety and devotion, yet the sincere Christian is to be abundantly satisfied with God’s approbation of his services, and with the silent applause of his own conscience.