Purpose is wrong. Being done out of obedience to some religious leader, instead of doing it from heartfelt reasons. The method is wrong. Giving up watermelon in February is no real sacrifice. Giving up all food is. That is true sacrifice. “My knees are weak through fasting; and my flesh faileth of fatness.” (Psalms 109:24).
Ashes on the forehead is wrong. Some claim it to be, “a personal act of remembrance and as a sign or a witness for others.” However, Jesus commanded, “But thou, when thou fastest, anoint thine head, and wash thy face; that thou appear not unto men to fast, but unto thy Father which is in secret: and they Father, which seeth in secret, shall reward thee openly.”
Only one day! Jesus left his work went into the wilderness and fasted for 40 days! (Matthew 4:1,2).
Ryan Duncan adds, “All too often, Christians (and I’m including myself here) use fasting as a ploy. We try to draw attention to ourselves, or a cause we’ve stepped behind, or we simply want to believe we’re spiritually stronger than we truly are. But a selfish heart will never yield humility. By its very nature, humility seeks no personal gain, but strives to put God first. That’s why so many attempts at fasting fail, because we’re secretly doing it for ourselves rather than Christ.”