Fasting

Fasting is good training in self-denial. It's an exercise in which we choose to give precedence to the life of the Holy Spirit over our physical needs. 

Biblical fasting should be done in secret (Mat 6:16–18). It's a way to humble myself and to seek God in prayer (Psalm 35:13, Joel 2:12–13). Fasting should always be coupled with prayer (Acts 13:3). It's about setting aside ordinary daily life so that we can focus on prayer. From a Biblical perspective, fasting is not eating food and only drinking water. We can fast that way but also "fast"
(self-denial) from other things that take up our time, like television and movies. The purpose is to make Jesus the focus of this period, see
time 30:26–36:34 in Luke 22 (Part 4) :31-46 • Standing in the Hour of Temptation.

Fasting was originally an expression of mourning, and Biblical fasting was originally a way to show God how to grieve over sins committed. John the Baptist 's disciples fasted and it can be seen as part of their repentance, see also The Book of Jonah. However, fasting wasn't part of the Mosaic Laws, but had been part of the Jewish understanding of the Law. Over time, this fast had turned into a purely religious ritual, something that was done routinely without really thinking about the meaning (Isaiah 58:3–14). It had become tradition. The Pharisees regularly fasted two days a week and many of them (perhaps most) did so primarily to gain attention and admiration for their "deep piety", see The Pharisee and the Tax Collector and 
Jesus Denouncement of the Jewish Religious Leaders.

Jesus' disciples didn't fast during Jesus' ministry, which resulted in criticism of Jesus. Jesus' answer wasn't that there was anything wrong with fasting. But fasting must be honest, not just an empty ritual. It must be an expression of sincere sorrow, otherwise it's meaningless. It must come from the heart. Exteriors and empty rituals mean nothing. God only looks at the heart. Ceremonies and rituals aren't crucial. 

There are therefore suitable times for fasting and times when one shouldn't fast. Jesus' disciples didn't fast as long as Jesus lived among them. Jesus said: "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"

(Mat 9:14–15a, Mark 2:18–19, Luk 5:33–34).

Fasting in pure routine, despite being happy, isn't honest fasting. Fasting when not mourning is as absurd and inappropriate as dressing in sackcloth and ashes and fasting at a wedding feast, see 
The Parable of Putting New Wine into Old Wineskins.

Jesus also said: "The days will come when the bridegroom is taken away from them, and then they will fast." (Mat 9:15b, Mark 2:20, Luk 5:35). Jesus' disciples fasted later, after Jesus' ascension into Heaven (Acts 13:2–3, 14:23).

See also Some stories in the gospels at the bottom of The Four Gospels.