Jesus did not come to patch up people’s practice of the law. He came to fulfill the law and help us understand it in its proper context. Jesus gives John’s disciples a revelatory answer to their question about the discipline of fasting. Jesus tells them (in not so many words) that there will be plenty of time for fasting once he has gone to the cross and died. The crucifixion of Jesus will be an appropriate event for fasting. Just as new wine must be put into new wineskins, so Jesus comes to put spiritual disciplines and faithful devotion in its proper context. Fasting and other disciplines are an expression of a changed heart and mind, not simple adherence to a external ritual.
Matthew 9:14-17 (ESV)
14 Then the disciples of John came to him, saying, “Why do we and the Pharisees fast, but your disciples do not fast?” 15 And Jesus said to them, “Can the wedding guests mourn as long as the bridegroom is with them? The days will come when the bridegroom is taken away from them, and then they will fast. 16 No one puts a piece of unshrunk cloth on an old garment, for the patch tears away from the garment, and a worse tear is made. 17 Neither is new wine put into old wineskins. If it is, the skins burst and the wine is spilled and the skins are destroyed. But new wine is put into fresh wineskins, and so both are preserved.”