"And whenever you fast, do not look dismal, like the hypocrites, for they disfigure their faces so as to show others that they are fasting. Truely I tell you, they have received their reward. But when you fast, put oil on your head and wash your face, so that your fasting may be seen not by others but by your heavenly Father who is in secret; and your Father who sees in secret will reward you." (Matthew 6:16-18) (NRSV)
We move into a series of sayings on fasting, treasure, and allegiance. Let's take up fasting in this post.
Fasting was an acceptable act of piety in Jesus' day. To the best of my knowledge, he had no issue with fasting per se. He was less enthralled with what he felt to be a self-defeating form of public fasting. Perhaps with a bit of exaggeration, Jesus paints a picture of someone who alerts others that he or she is fasting by assuming a gloomy look or decorating their face with ashes, paint or such. I don't believe Jesus cared one way or the other about the use of ashes or other facial decoration. His question was: For whom or for what reason are you fasting?
His targets fasted in order to get public credit for the act. More subtly, they fasted in order to gain a kind of power over those who observed them. In such cases, fasting became not a tool for spiritual liberation but a means of binding oneself even more tightly to the "normal" human game of social manipulation.
Jesus opted for private fasting. While one might well go out in public while fasting, one's appearance and behavior ought to give no clue that one was fasting. To Jesus way of thinking, fasting was meant to be a tool that might free us to better experience or focus on God.
That's the point of the language about God seeing what is done in secret and rewarding us. The reward is not something postponed until some judgment day, but instead comes in bits and pieces, and occasional transcendent moments, as we experience liberation from preoccupation with self coupled with a sharp awareness of the reality/presence of God.