There is a hymn from the 1940 Hymnal whose lyrics capture the Scriptural precedence for fasting quite well. At the end of the lyrics I'll give an exposition on some of the lines and how what we sing actually shapes what we believe. Some of us at St. Aelred like to sing this one at Morning or Evening Prayer. I especially like it because it seems to accord with The Catechism nicely, "CCC#1158: "But 'the texts intended to be sung must always be in conformity with Catholic doctrine. Indeed they should be drawn chiefly from the Sacred Scripture and from liturgical sources."" You don't find many praise songs doing this, but hymns do!
The glory of these forty days
We celebrate with songs of praise;
For Christ, by whom all things were made,
Himself has fasted and has prayed.
Alone and fasting Moses saw The loving God who gave the law;
And to Elijah, fasting, came The steeds and chariots of flame So Daniel trained his mystic sight, Delivered from the lions' might And John, the Bridegroom's friend, became The herald of Messiah's name Then grant us, Lord, like them to be Full oft in fast and prayer with thee; Our spirits strengthen with thy grace, And give us joy to see thy face. O Father, Son, and Spirit blest, To thee be every prayer addrest, Who art in threefold Name adored, From age to age, the only Lord. Amen. (Latin tune, 6th c.)
We should fast a full 40 days. The glory of these forty days: Fasting on only one day, or only on Fridays is a handful of days, but it is not fasting for 40 days. The season is penitential, not a handful days in the season. This is what Christ did, and what we proclaim that Christians are doing in Lent. So let's do it! If you haven't decided on a Lenten Fast yet it's not too late! Read Matthew 6 and see how Jesus does not say "if," but "when" you practice almsgiving, praying, and fasting do so not to be seen but for the sake of God.
We should fast food, not fast candy and eat fastfood. For Christ, by whom all things were made,Himself has fasted and has prayed. Contrary to popular myth that "Catholic don't need to fast after Vatican II," Christ himself fasted, and he is God. They didn't over spiritualize this, they meant food. How much more should we?
We learn to love The Lord's Law by cleansing our desires of ill loves. Alone and fasting Moses saw // The loving God who gave the law We remember Moses brought down The Commandments, we rarely remember he was up there for 40 days fasting! Thus Christ's 40 day wilderness fast also fulfills Moses' own. "And he was there with the Lord forty days and forty nights; he neither ate bread nor drank water. And he wrote upon the tables the words of the covenant, the ten commandments." (Exodus 34:28)
Fasting is part of "getting to heaven." And to Elijah, fasting, came // The steeds and chariots of flame Again, we might recall the flaming chariot that picked up Elijah (2 Kings 2), but we rarely remember it was while he was fasting. Thus Elijah's fast was a foreshadow of Christ's own, and his going up on a flaming chariot was but a type for Christ's Ascension. "And he arose, and ate and drank, and went in the strength of that food forty days and forty nights to Horeb[a] the mount of God." (1 Kings 19:8)
Fasting is part of training one's eyes to see truth. So Daniel trained his mystic sight, Daniel's "fast" meant no meat and no wine. Beware 'The Daniel Fast' today; it's a diet, not a fast, which means it's ultimately about us, not God. "In those days I, Daniel, was mourning for three weeks. I ate no delicacies, no meat or wine entered my mouth, nor did I anoint myself at all, for the full three weeks." (Daniel 10:2-3) This is not 40 days, but a 3 week fast. Nevertheless we learn that all of our righteous forefathers fasted. Also, "mystic sight" is something that is quite difficult to explain. Christ frequently says "though seeing they do not see." There is a clarity of mind that comes about from a fast which gives spiritual insight into oneself and others. It is beyond words.
Christ said we would Fast after he Ascended. And John, the Bridegroom's friend “'The disciples of John fast often and offer prayers and so do the disciples of the Pharisees, but yours eat and drink.' And Jesus said to them, 'Can you make wedding guests fast while the bridegroom is with them? The days will come, when the bridegroom is taken away from them, and then they will fast in those days.”" (Luke 5:33) Since Christ has ascended, we are now "without the bridegroom." Thus now is the age to fast, and Lent is the season.
The entire Church is fasting and praying, ours is a part of a team effort. Full oft in fast and prayer with thee; The hymn concludes that what they all have in common is fasting and prayer. Thus we take to both as well. Recall from Matthew 6 we as individuals are not to tell others of our fast, but as a community, as a whole, we are free to so that we can help one another grow spiritually (St. Frances de Sales). Because we are all members of the body of Christ, anything we do is part of that body acting. There is no such thing as "private" prayer or fasting then, only "personal" prayer or fasting that assists in Christ's salvific work. We're never alone!
Lent is The Desert, the eucharist is our manna, and Easter is the Promised Land. Our spirits strengthen with thy grace, We learn that fasting is not willing oneself to salvation (Pelagianism), but relies upon God's grace at the source and as the summit of the Christian life. This points to the eucharist which becomes our spiritual food, our Manna in the wilderness.
The Promise for the Penitent is Eternal Rest from their Labors. And give us joy to see thy face. This is the Christian Hope and our ultimate goal for fasting. We hope to enter into God's Rest one day, which he promised to us, just as he promised rest to The Israelites. But also compare this to The Book of Exodus and Psalm 95 The Venite which recalls the Exodus story. There God tells the impenitent Israelites, "that they should not enter into my rest." Penance is thus necessary in order to be forgiven. God will not force his forgiveness upon us, but requires and awaits a response, as a Lover to a Beloved.
Spiritual Joy is cultivated and happy, Fake Joy is lazy and angry. And give us joy to see thy face. This also shows that joy is a fruit of discipline. Many who want spiritual joy attempt to clutch it tightly in their hands as if one could "choose" joy. When one wants apples, they begin tilling the soil to plant. The rain must first come, only then can the seed open and become an apple tree. So too almsgiving, prayer, and fasting is our response to the rains of The Lord's grace upon the garden of our hearts. Too many want to talk about having "Christian Joy" but have no disciplines. This is not True Devotion, but False Devotion. This is a "joyful passion," but it is not "spiritual joy," which is about God proper. False Devotion often appears as being "Nice." One wrong word turns their nicety into Wrath. This is why I frequently say "Nice is a Vice." True Devotion is joy when working in Our Father's field. Spiritual Joy is not a virtue, but it is a fruit of Charity, a sort of "bonus" if you will, which is a sign of spiritual flourishing, just as fruit on a vine signals the vine is flourishing.