Now John’s disciples and the Pharisees were fasting. And people came and said to him, “Why do John’s disciples and the disciples of the Pharisees fast, but your disciples do not fast?” And Jesus said to them, “Can the wedding guests fast while the bridegroom is with them? As long as they have the bridegroom with them, they cannot fast. The days will come when the bridegroom is taken away from them, and then they will fast in that day.
Mark 2: 18-20 ESV
Fasting is only told a couple of time in the Old Testament book Leviticus and it’s prescribed for day of atonement. It is done as a symbol of sorrow of sins to show penitence. However in the time of New Testament, among the Jewish religious fasting is a normal tradition and it’s done twice a week, on Mondays and Thursdays. The people were astonished that Jesus is not following such tradition, instead He answered with an analogy of the bridegroom. That the bridegroom is now here, so the wedding guests should not fast, because it’s the time for feasting instead. However there will come a day when the bridegroom will be taken away… By saying this analogy Jesus is prophesying about his own death.
His coming brings the grace of God by giving us salvation therefore we should rejoice!