Jamieson, Fausset and Brown on Matthew 6:16

16. Moreover, when ye fast--referring, probably, to private and voluntary fasting, which was to be regulated by each individual for himself; though in spirit it would apply to any fast.

be not, as the hypocrites, of a sad countenance: for they disfigure their faces--literally, "make unseen"; very well rendered "disfigure." They went about with a slovenly appearance, and ashes sprinkled on their head.

that they may appear unto men to fast--It was not the deed, but reputation for the deed which they sought; and with this view those hypocrites multiplied their fasts. And are the exhausting fasts of the Church of Rome, and of Romanizing Protestants, free from this taint?

Verily I say unto you, They have their reward.