The first half of Matthew six describes the righteous in action—they serve others; they pray; they fast (vv. 1-18). In verses 16-18, Jesus says, “Moreover, when you fast, do not be like the hypocrites, with a sad countenance. For they disfigure their faces that they may appear to men to be fasting. Assuredly, I say to you, they have their reward. But you, when you fast, anoint your head and wash your face, so that you do not appear to men to be fasting, but to your Father who is in the secret place; and your Father who sees in secret will reward you openly.” Note these three specifics about fasting:
Do fast. Though abstaining from food for a given period for spiritual reasons and purposes is never specifically commanded or forbidden in the New Testament, Jesus does state his expectation of it. In Mark 2:18-20, the disciples of John the Immerser and the Pharisees were fasting. They came with the question, “Why doesn’t your disciples fast?” Jesus response was, “Can the friends of the bridegroom fast while the bridegroom is with them? As long as they have the bridegroom with them, they cannot fast. But the days will come when the bridegroom will be taken away from them, and then they will fast in those days” (vv. 19-20). Clearly, Jesus expects His disciples to fast based upon their spiritual and emotional needs, not based upon the day of the week (as the Pharisees did).
Do not fast to be seen of men. Fasting is not a performance. It is not a meritorious work which earns you spiritual extra credit. It ought to never aid or abet a self-righteous spirit or the praise of men.
Do fast to be seen of God. Like prayer (v. 6), fasting is best done in secret.