And they said to Him, “The disciples of John often fast and offer prayers, the disciples of the Pharisees also do the same, but Yours eat and drink.” And Jesus said to them, “You cannot make the attendants of the bridegroom fast while the bridegroom is with them, can you? But the days will come; and when the bridegroom is taken away from them, then they will fast in those days.”
Luke 5:33-35 (NASB)
“Whenever you fast, do not put on a gloomy face as the hypocrites do, for they neglect their appearance so that they will be noticed by men when they are fasting. Truly I say to you, they have their reward in full. But you, when you fast, anoint your head and wash your face so that your fasting will not be noticed by men, but by your Father who is in secret; and your Father who sees what is done in secret will reward you.
Matthew 6:16-18 (NASB)
I added the bolded letters. This sounds like a pretty definite when statement to me. When you fast, not if.