Christians observe special times and season for the disciplines of fasting, keeping vigil, and other forms of enacted prayer. It is also appropriate to observe these disciplines at any time, especially in preparation for specific acts of discipleship or as acts of penitence, reconciliation, peacemaking, social protest, and compassion.
Sadly, I can find nothing else in the Directory for Worship nor the Book of Common Worship regarding fasting. There are some fine instructions on family prayer and devotional that are worth looking at, however.
Another resource is simply in reflecting upon the great ends of the church, as expressed in chapter 1 of the Book of order:
* The proclamation of the gospel for the salvation of humankind
* The shelter, nurture, and spiritual fellowship of the children of God
* The maintenance of divine worship
* The preservation of the truth
* The promotion of social righteousness
* The exhibition of the Kingdom of Heaven to the world
Evangelism, mission, discipleship, holiness, truth, fellowship -- these all sound like fine things to focus upon in the name of Jesus Christ.
Soli Deo Gloria